Mistletoe, Past and Present

"Everybody knows, a turkey and some mistletoe…" (1)

Leafy mistletoe in the crown of a willow oak.
Photo by Dr. Larry F. Grand

The age-old tradition of kissing under the mistletoe is well known in our culture, and most people have heard of the importance of mistletoe to the ancient Druids, for whom it served ritual and medicinal purposes (2). Many also know the berry-like fruits to be toxic to people, but according to Poisonous Plants of North Carolina, poisoning will occur only if large quantities are eaten.

Fewer people know the marvelous mistletoe story from Norse Mythology, wherein a conniving Loki arranges for a mistletoe arrow to be used to kill Balder, whom the creatures of earth were sworn not to harm. The mistletoe – which never touches the earth – was not obligated by the oath (3). On a more scientific note, mistletoe was recognized as being a parasite of its host tree as far back as the 13th century by Albertus Magnus (4), a.k.a. St. Albert the Great, who gets my vote for patron saint of plant pathology.

In North Carolina mistletoe is a common sight this time of year in oaks and other hardwoods, since it stays green even after its host tree has dropped its leaves. I've seen mistletoes recently in sycamore and even in ornamental pear. They usually appear as round sprays of leafy stems high in the crown. This makes them inconvenient for gathering as holiday decor, and people have been known to bring them down with a shotgun blast. In some cases – this seems especially common in red maple – mistletoe occurs as a trunk infection, causing a gnarling and roughening of the bark. The mistletoe shoots coming from these trunks are more accessible to would-be collectors, but are rather too short for effective decorating. Unless someone is beating me to the bigger ones.

Leafy or true mistletoes in the New World belong mostly to the genus Phoradendron. They are classified in the Viscaceae, the same family as the famous Eurasian mistletoe, Viscum album. Both are "water parasites" which take up water and dissolved minerals from their host trees but photosynthesize most of their own food. As you might imagine, this arrangement is a more serious burden for trees in the drier parts of the country than here in the Southeast. You would not likely confuse the true mistletoes with their cousins, the dwarf mistletoes (Arceuthobium spp.). Not only are their size and appearance different, but dwarf mistletoes don't occur in North Carolina, even in the seemingly suitable climate of our mountains. They do occur in western and northern U.S. states, where they can be serious pests of conifers. 

Dwarf mistletoe. Photo by David Shew

If you look up mistletoe in a Spanish-English dictionary, you'll be given the word "muérdago", but in my wife's home in southern Mexico, mistletoe is referred to as "injerto". This word means "graft" and is a very apt moniker, given the xylem union between the mistletoe and its host. Interestingly, the mistletoe of my wife’s upbringing has showy flowers, much different from the inconspicuous flowers of our local types.

The mistletoe's whitish berries are eaten by birds, but the seeds pass through the birds’ digestive systems. Some are lucky enough to be deposited on thin-barked tree branches, where they are held by the seeds' sticky coating (Remember the family name Viscaceae?). After germination, instead of roots the young parasite forms structures that penetrate the branch and establish an infection. The mistletoe's shoots develop later, but are very small the first year. Growth continues in subsequent years, and the mistletoe will be with the branch for life. (6)

"Your heart's a dead tomato / splotched with moldy purple spots, Mr. Grinch." (7)

From all of us here at the Plant Disease Clinic, Happy Holidays! Please remember that we'll be closed December 26-30, 2011. We look forward to checking your tomatoes, trees, and the critters that bug you in 2012.

References
(1) Mel Tormé and Bob Wells. 1944. "The Christmas Song".
(2) Bussing, Arndt, ed. 2000. Mistletoe: The Genus Viscum. Harwood Academic Publishers. p.1
(3) Ibid., p.2
(4) Agrios, G. 1997. Plant Pathology, 4th ed. Academic Press. p.10
(6) Sinclair, W.A., and Lyon, H.H. 2005. Diseases of Trees and Shrubs, 2nd ed. Cornell University Press.
(7) Theodore "Seuss" Geisel. 1966. "You're a Mean One, Mr. Grinch".

Sample of the week: Black rot of crucifers

If you are looking forward to a nice mess of collards for Christmas or greens for the first meal of the New Year, this week’s samples are for you. We received several samples of seedlings and plants in the cabbage family that had yellow or brown yellow lesions. Some of the leaves had V-shaped lesions along the margins, while other leaves also had necrotic and blackened veins.

Black rot symptoms caused by Xanthomonas campestris pv. campestris. Note V-shaped lesions on leaf margins.

Collard leaves with blackened veins. Water droplets formed at the hydathodes can be seen along the margins of the upper leaf. Photo by Lisa Rayburn.

These symptoms are typical of black rot, a common disease of crucifers (cabbage, broccoli, kale, collards, turnips, etc.) caused by the bacterium Xanthomonas campestris pv. campestris. Bacterial ooze was visible coming out of the lesions on seedlings, supporting the black rot diagnosis.

The black rot bacterium infects plants through wounds and natural openings, such as hydathodes (water pores) found on the leaf margins. Later, the bacteria invade the vascular system and spread systemically. The disease is favored by mild, wet weather and the bacteria can rapidly spread from plant to plant in splashing rain. Bacteria can be spread when plants are handled (for example, during transplanting) and they survive on and in seed and in debris in the soil. 

Black rot is very difficult to control because it is systemic and because we have very few chemicals that are effective against bacteria.  To reduce spread, avoid cultivation or handling when plants are wet. To prevent black rot, plant clean seeds or seedlings into areas where no crucifers have been grown for 2-3 years.

For more information see:

http://www.ces.ncsu.edu/depts/pp/notes/oldnotes/vg16.htm

http://www.extension.org/pages/29925/managing-black-rot-of-cabbage-and-other-crucifer-crops-in-organic-farming-systems

Poinsettia: Some Common Diseases of the Christmas Flower

Fig. 1. Poinsettia- the Christmas flower
(with permission Benson, et al. 2002. Plant Health Progress
doi:10.1094/PHP-2002-0212-01-RV).

Poinsettia, the Christmas flower, (Fig. 1) was introduced to the United States from Mexico in 1825 by the first U.S. Ambassador to that country, Joel Roberts Poinsett of Greenville, South Carolina (Fig. 2). 

Fig. 2. Joel Roberts Poinsett, first US Ambassador to Mexico
(with permission Benson, et al. 2002. Plant Health
Progress doi:10.1094/PHP-2002-0212-01-RV).

Commercial interest in poinsettia as a potted plant grown in the greenhouse did not get much attention until the 1950s and 60s, when breeding programs developed plants with stiffer stems, multiple shoots at each pinch point, larger flower bracts, and better keeping qualities. Today, poinsettias come in a variety of forms and bract colors. The value of poinsettias is about $145 million per year in the United States with about $17 million coming from North Carolina growers (Fig. 3). Although poinsettias are the Christmas flower, it is only consumer preference that limits year round sale.

Fig. 3. Poinsettias in commercial production
as the flower bracts are beginning to turn red.

Poinsettias are propagated vegetatively by cuttings taken from stock plants usually beginning in late June and early July just when greenhouse temperatures are highest. Cuttings are propagated typically in either polyfoam wedges, rockwool, or direct stuck in the finish size pot. Regardless of propagation strategy, cuttings must be misted several times a day to keep them from wilting until roots form on the stem of the cutting (Fig. 4). 

Fig. 4. Propagation of poinsettia cuttings in polyfoam rooting
wedges under an intermittent mist system.
Note droplets of water on foliage from misters.
(Photo E. Lookabaugh) 

During propagation, growers must avoid or prevent a number of plant diseases that can attack the cuttings. Under extreme moisture conditions, the soft rot bacterium, Erwinia carotovora attacks the cut end of the stem resulting in a mushy, watery rot that kills the cutting (Fig. 5). 

Fig. 5.  Erwinia soft rot has collapsed these poinsettia
cuttings in propagation
(with permission Benson, et al. 2002. Plant Health
Progress doi:10.1094/PHP-2002-0212-01-RV).

Even when misting systems are functioning normally, Rhizoctonia stem rot caused by R. solani can cause a canker on the lower stem that kills the cutting (Fig. 6, 7). When cuttings are stuck directly in potting mix in the finish pot, Pythium rot caused by several species of Pythium as well as Rhizoctonia stem rot can develop, if these pathogens are introduced by faulty sanitation procedures. Healthy cuttings root in about 4 to 6 weeks depending on temperature, if plant diseases do not develop.

Fig.  6. Poinsettia cutting in a polyfoam propagation
 strip with Rhizoctonia stem rot.  Note brown stem
 lesion at bottom of cutting near foam surface. (Photo Mike Benson)

Fig. 7. Rhizoctonia stem rot. Two close ups of a stem lesion with
the white mycelium of the Rhizoctonia pathogen present
(with permission Benson, et al. 2002. Plant Health
Progress doi:10.1094/PHP-2002-0212-01-RV).

Cuttings once rooted in polyfoam wedges or rockwool must be transplanted to a soilless potting mix in a pot to finish for retail. The most important foliar disease growers must guard against in this stage of production is gray mold caused by Botrytis cinerea.(Fig 8). As the plant canopy grows and fills in, high humidity in the microclimate of the canopy is an ideal environment for gray mold. Growers must ventilate greenhouses properly to avoid high humidity and some even use bottom heat via air tubes under the greenhouse bench to help dry out the plant canopy. Fungicide sprays may also be used to prevent gray mold.

Fig. 8. Botrytis blight on foliage. Note dead tissue and abundant
 ‘gray mold’ sporulation on the infected tissues. This infection developed
 inside the plant canopy where humidity was high favoring
pathogen infection and sporulation. (photo Mike Benson)

In the 1990s powdery mildew caused by Oidium spp. caused severe losses for many growers. The disease was particularly devastating because it often times did not develop until the plants already had color in the flower bract and by that time the grower had most of the expense of growing the crop already invested in it (Fig. 9). Growers also were reluctant to use fungicides sprays for powdery mildew control in the late stages of production because of spray residue concerns on the flower bracts. The disease has not been a problem in the last decade, however, due to changing cultivars and better management practices.

Fig. 9. Colonies of powdery mildew on leaves (left) and flower bracts (right)
(with permission Benson, et al. 2002. Plant Health Progress doi:10.1094/PHP-2002-0212-01-RV).

Scab is a stem and foliage disease caused by the fungus Sphaceloma poinsettiae that can occur periodically, resulting in unsalable plants. The most striking symptom of scab is the extra long stems produced by plants infected with this fungus (Fig. 10). Leaf spots also develop on infected plants. Outbreaks of scab usually occur when the pathogen is introduced with poinsettia stock material arriving from Central and South America where the fungus occurs throughout the year.

Fig. 10. Abnormally elongated stems of poinsettia due to scab disease
(with permission Benson, et al. 2002. Plant Health Progress doi:10.1094/PHP-2002-0212-01-RV).

The most important root disease affecting poinsettia is Pythium root rot caused by several species of Pythium the most common being P. aphanidermatum, but P. irregulare, P. cryptoirregulare and P. ultimum also cause loss. The fungus-like Pythium survives between crops in infected plant material from previous crops whether they are poinsettia or not. Without thorough sanitation between crops Pythium can be re-introduced to the new poinsettia crop by infested crop debris or through the irrigation system. The most common symptom of Pythium root rot is stunting of the plant as it fails to keep pace with the growth of healthy plants (Fig. 11). 

Fig. 11. Stunting of poinsettia plants caused by Pythium root rot during finishing.
 Note healthy plant in foreground compared to stunted, disease plants scattered throughout.
(Photo Mike Benson)

Under severe disease pressure, the foliage of plants with Pythium root rot develops wilt symptoms and does not recover with irrigation. Affected roots are discolored (Fig. 12). This disease can attack the crop at any time from propagation through finishing. Pythium root rot occurs in greenhouses regardless of location as some Pythium species are aggressive at low temperatures and others at high temperatures. Overwatering favors Pythium root rot. Fungicide drenches are commonly used to prevent the disease.

Fig.  12. Pythium root rot of poinsettia on a newly-transplanted rooted cutting. 
Wilt symptoms (left) and close up of discolored roots with root rot from same plant (right).
(Photo Mike Benson)

Phytophthora root rot caused by P. drechsleri and P. nicotianae also can attack poinsettia during the finishing stage resulting in unsalable plants. Phytophthora is fungal-like pathogen similar to Pythium. Symptoms are the same, as well. Unlike Pythium, however, these Phytophthora pathogens can also splash onto poinsettia foliage causing a blight disease. Like Pythium root rot, overwatering favors this disease too and fungicides are commonly used to prevent the disease.


For a detailed history of the poinsettia and poinsettia diseases click here

Post prepared by Mike Benson

Sample of the Week: Girdling on Arizona Cypress

Notice constricted base (Photo: Jennifer Pries)

This week we received an Arizona cypress that had died back rapidly in the fall. Nearby trees were unaffected and the client suspected that the lower stem might have been damaged by voles. However, Dr. Hodges, our tree diagnostician, found constriction at the lower stem (trunk) but could not find evidence of vole damage. Upon further examination, Dr. Hodges concluded that a root had grown around the base of the stem, girdling the tree. Root girdling occurs when the roots of the plant grow around the base or main stem of the tree causing restricted water and nutrient movement. Growth slows down, leaf production decreases and eventually, the tree will begin to die back.

Notice roots curling around base (Photo: Jennifer Pries)

Planting trees too deep or mulching too high around the tree can promote root growth around the base of the tree. Trees grown in container nurseries are more prone to root girdling because the roots are forced to grow in a circular fashion in the pots. Proper pruning of the roots in the nursery and before transplanting can significantly reduce the potential for root girdling in future.


For more information on proper transplating, click here
For more information on root girdling, click here

Edema

This week’s sample consisted of several detached leaves from a spathe flower plant. Symptoms consisted of chlorotic blotches and some darker flecking on the leaves. One leaf was completely brown but not decayed. The symptoms were not suggestive of a foliar disease, and no fungi were observed on the spots. As a precaution, some leaf pieces were incubated, but nothing grew out.

Edema on spathe flower (Photo: Mike Munster)

Under the microscope, there were small, slightly raised bumps on the leaves. Mike Munster, the diagnostician who worked with the sample, suspected edema. Edema is a common abiotic disorder that occurs on many herbaceous and woody plants. Edema can be a problem on greenhouse crops and on plants grown outdoors. The most obvious symptoms include the formation of tiny blisters, warts, or raised bumps on the undersurface of leaves. With age, these bumps may become reddish brown and can be mistaken for rust pustules.

Notice red pustules on underside of leaf (Photo: Mike Munster)

Edema on geranium (Photo: Mike Munster)

Typically, edema is a problem in cool, wet conditions when the soil water is warmer than the atmosphere. Edema occurs when the roots take up more water than is lost through transpiration, resulting in accumulation of water in intercellular spaces. Excess water accumulates in the leaf causing leaf cells enlarge and block the stomatal openings. These enlarged inner cells cause the epidermis to rupture and become crusty with age.

Crusty edema on tomato (Photo: Frank Louws)

Crusty edema on tomato (Photo: PDIC Database)

For control:

• Avoid irrigating during cool, humid, cloudy weather.
• Reduce humidity in greenhouses by venting and increasing heat, improving air circulation, increasing light intensity, spacing plants further apart.
• Avoid standing water under pots

Edema on swiss chard, notice the blackened stomata (Photo: Mike Munster)

Edema on lilac, similar to fungal leaf spots (Photo: Mike Munster)

For more information, click here

Phytophthora Root Rot on Fraser Fir

Fraser Fir: The "Cadillac" of Christmas Trees

Fraser fir is a highly desirable Christmas tree species grown in the mountains of Western North Carolina and elsewhere across the United States. As a leading producer of Christmas trees, North Carolina growers must overcome weeds, insects, and disease to produce that beautiful tree you buy on the Christmas tree lot. 

Fraser fir nursery bed where seedlings are grown up to 

5 years before transplanting to the field.  (Photo: Mike Benson)

Typical Fraser fir planting in Western North Carolina 

after about 5 to 6 years in the field. (Photo: Mike Benson)

Although growers have good management tools for weeds and insect pests, Phytophthora (pronounced fy-TOF-thor-uh) root rot threatens long-term production of Fraser fir. The pathogen, Phytophthora, produces zoospores that can swim to tree roots in saturated soil or run-off. Throughout the 11 to 15 years or more that is takes to produce a 6 to 8 foot tall Fraser fir, Phytophthora root rot is always a threat. 

Fraser fir planting that experienced Phytophthora root rot that started on trees at top of 

ridge. Over several years, trees downhill from ridge became infected and died.  

The Phytophthora pathogen produces spores that are carried in water run-off, so

the ‘wedge’ shaped area of missing trees developed in the downslope drainage 

area of the field as trees were killed by the disease.  (Photo: Mike Benson)

Fraser fir planting  after about 6 years in the field. Note patch of trees killed by 

Phytophthora root rot in far edge of field. (Photo: Mike Benson)

Lower branch ‘flagging’ may be the first indication that a Fraser fir has Phytophthora root rot.  Once the Phytophthora pathogen has infected enough of the root system that the plant can no longer transport water and nutrients  adequately, the foliage becomes yellow then turns reddish brown a short time later.  The tree will eventually die from the disease. 

Lower Branch "Flagging" (Photo: Mike Benson)

Root systems from a healthy Fraser fir transplant (right) compared to an infected 

and dying transplant (left).  Roots killed by the Phytophthora root rot pathogen 

are dark, reddish brown in comparison to root tips on healthy plants that are white. 

(Photo: Mike Benson)

Growers manage the disease by planting healthy fir transplants into fields that are well drained. Once disease develops in a tree, however, that tree is lost. Future production in that area of the field is also threatened because the pathogen survives many years in soil.

In the future, forest tree breeders and plant pathologists hope to develop a Phytophthora resistant fir with qualities equal to the Fraser fir. So when you buy that fresh cut Christmas tree this year, just remember that the Christmas tree grower was able to overcome a lot of potential problems to provide that tree for you.

For more information, click here

Special thanks to Mike Benson for writing this post!

Sample of the Week: Papaya Ringspot Virus on Watermelon

Recently, we received a very interesting watermelon sample that was infected with Papaya Ringspot Virus (PRSV).  This is only the second time we have seen this disease in the clinic.  PRSV causes diseases on cucurbits and papaya plants worldwide.  The most obvious symptom of PRSV infection is the formation of sunken ring spots on the fruit.  These spots are very diagnostic of the disease.  Other symptoms include mosaic or mottling of the leaves and oily streaks on the stems and petioles. In some cases, the leaves may be severely narrowed, which gives them a “shoestring” like appearance. 

Rinspot symptoms of PRSV (Photo by: Jennifer Pries)

PRSV is a member of the potyvirus group of viruses and is transmitted by aphids. The virus is vectored in a nonpersistant manner, meaning it does not replicate in the aphid vector.   Aphids feed on infected plants, and within seconds or minutes they hop to healthy plants and transmit the virus as they feed.  The virus can spread very quickly through fields. 

The two major strains of PRSV are PRSV-W and PRSV-P.  PRSV-W (the watermelon-infecting type) used to be known as Watermelon Mosaic Virus I, and can only infect cucurbits.  PRSV-W is the major strain found in the Southeastern US, but it is also present in tropical areas where PRSV-P is present.  PRSV-P (the papaya-infecting type) is only found in tropical and subtropical areas were papayas are grown, but PRSV-P can also infect cucurbits.  Controlling this disease is somewhat difficult, especially in papaya.  Genetic resistance to PRSV is available in some commercial varieties of cucurbits and is the most effective way of controlling the disease.  Unfortunately, resistance has not been found in watermelon.  Aphid  control is not practical because of the nonpersistant manner in which the virus is transmitted.  

For more information,

http://www.apsnet.org/edcenter/intropp/lessons/viruses/Pages/PapayaRingspotvirus.aspx

http://www.extento.hawaii.edu/kbase/crop/type/papring.htm

For resistant varieties,

http://vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm

Sample of the Week Sweetpotato Scurf

With Thanksgiving on the way, we decided to highlight a common disease of sweetpotatoes, but don’t worry, it wont affect the taste of your sweetpotato casserole! Scurf is caused by the fungus Monilochaetes infuscans.  The fungus only grows in the sweetpotato skin and produces spores on the surface of the skin.  As shown in the photos, the dark splotches on the skin are areas infected with the fungus.  The fungus does not penetrate below the skin, so the sweetpotatoes are still edible, but because of the dark discoloration that results, diseased sweetpotatoes have lower market values. 

Healthy sweetpotatoes are infected by spores present in the soil.  The disease can also spread through contaminated crates, baskets, and storage houses.   The fungus does not attack the above-ground portion of sweetpotato plants, and does not attack other hosts so rotation is an effective way to prevent scurf problems.  Scurf is usually worse during rainy seasons.  

For more information and control recommendations, click here

Sample of the Week: Cucurbit Downy Mildew on Greenhouse Cucumbers

This week’s sample is cucurbit downy mildew on greenhouse cucumber.  Cucurbit downy mildew affects all members of the cucurbit family: watermelons, cantaloupes, squash, pumpkins, and cucumbers.  Disease is favored by long periods of high humidity and mild temperatures, which unfortunately, describes most nights in the Southeast during the production season and in greenhouses year round.  Downy mildew occurs every year in North Carolina, and in recent years has become increasingly destructive on cucumber.  

Greenhouse Symptoms (Photo: PDIC Database)

The most obvious symptoms of downy mildew are small angular spots on the foliage, with older leaves generally being infected first. “Angular leaf spots” describe a type of symptom where the spots are defined by the boundaries of leaf veins, so that the shape of the spots is angular rather than round or blotchy. Downy mildew spots appear pale green to yellow as first, eventually becoming brown and necrotic with age.  

Angular Leaf Spots (Photo: PDIC Database)

During humid conditions, brown to purplish fuzzy growth can be observed on the underside of the foliage.  This downy growth is actually sporulation of the pathogen. 

Fuzzy Sporulation (Photo: Shawn Butler)

Dichotomously branched sporangiophores with lemon-shaped sporangia are visible when view through a microscope.  As infection progresses, leaves will eventually turn brown and curl upwards.  The leaves are the only part of the plant affected.  Downy mildew infections result in yield loss and misshapen fruits.  Damage from foliar infections also increases sunlight exposure on fruit and leads to sunscald.  

Cucurbit downy mildew is caused by Pseudoperonospora cubensis.  P. cubensis belongs to a group of fungus-like organisms called oomycetes (or water molds).  This group is also home to other aggressive plant pathogens including Phytophthora and Pythium.  P. cubensis is an obligate parasite, meaning it requires living host tissue to survive and reproduce.  North Carolina winter temperatures are too cold for the pathogen to overwinter, so it dies out every winter. Unfortunately, Southern Florida has the perfect combination of mild winters and wild cucurbit plants. The pathogen survives all year in Florida and inoculum builds up on wild plants.  Spores are windblown and can travel long distances on air currents. Summer weather events, like hurricanes, act as the perfect mode of transportation.  Like birds, downy mildew spores fly north for the summer and end up on our cucurbits! In this particular situation, the greenhouse plants probably became infected as spores from nearby field cucumbers blew into the greenhouse over the summer.

For more information on this disease, click here

For more information on controlling this disease, click here

For more information on forecasting this disease, click here 

Sample of the Week: Botrytis Blight of Pansy

Botrytis Blight on Pansy (Photo: Mike Munster)

This week we received pansies from raised beds in a trial garden in Henderson County, North Carolina. The roots and crowns were in good condition but some of the flowers, flower stalks and leaves were decaying. The main symptom on leaves was a very watery rot. The fungus Botrytis cinerea was observed sporulating on some of the material when we received the sample. Sporulation was heavy after a day of moist-chamber incubation, as indicated by distinctive masses of gray spores. Making a diagnosis when Botrytis is involved can be tricky, because the fungus can be both a secondary invader and a primary pathogen. It often infects spent blooms and then moves into other parts of the plant. In this case it appears to be fairly aggressive, so Botrytis blight is the diagnosis.

Wet rot of pansy leaves caused by Botrytis (Photo: Mike Munster)

Stereomicroscope view of pansy flower stalk with sporulation from Botrytis cinerea
(Photo: Mike Munster)

For more information on Botrytis Blight, click here
Thanks to Mike Munster for this "Sample of the Week" submission.

Household Molds: What Hurricane Irene Left Behind

Written by: Dr. Charles Hodges

A September 10, 2011 headline in the Raleigh News and Observer read “Irene leaves Aurora sodden in mold, misery”.  Similar headlines were probably common in other newspapers and on radio and television as Hurricane Irene took her heavy rains northward, resulting in flooded homes and businesses - and heavy mold development - in her wake.  

Mold development is a common occurrence whenever flooding occurs.  However, it does not take hurricanes and flooding to cause mold development in homes and other buildings.  Mold can develop anywhere there is adequate moisture and a suitable substrate on which to grow.  

Water from leaks in the roof and from broken pipes is the most common and serious cause of mold in buildings.  However, there are several other sources of moisture in homes that can result in the occurrence of mold.  Some of these include crawl spaces where the soil has become wet, poorly ventilated bathrooms, kitchens where steam from cooking condenses on the cabinets or walls, small closets in which are placed wet clothes or shoes, window sills wet by water that condensed on the inside of window panes, and condensation of moisture within HVAC ducts and on metal vents during the summer when the air conditioning is on.   

Two mold samples received in the Plant Disease and Insect Clinic during the winter came from old houses that did not have wall insulation.  The houses were occupied by elderly residents who kept the temperature very high, causing water to condense on the inside walls and leading to very heavy mold development.

Mold inside wall (Photo: PDIC Database)

Molds belong to a group of organisms called fungi.  There are many thousands of different kinds of fungi.  Some of these cause diseases of plants and animals, some are responsible for the decomposition of dead organic matter, and others are important in fermentation processes or in the production of antibiotics.  The basic vegetative phase of molds (threads called hyphae), and the reproductive phase (spores), are microscopic.  When large colonies of mold develop and produce spores, however, they are often visible to the naked eye.   The colonies can be of many different colors – white, black, blue, green, pink, or brown - depending on the individual mold species that produce them.  

Variety of household molds (Photo: PDIC Database)

Most molds are spread primarily as spores that move through the air on even the most gentle of air currents.  Upon landing on a suitable substrate, and with enough moisture, spores germinate and form a colony.  The rate of development of the colony depends on the individual mold species, the substrate, and temperature and moisture conditions.  Many of the most common household molds prefer substrates containing cellulose; e.g., the cardboard covering of sheetrock, ceiling tiles, wood, paper, and fabrics.  Dead leaves of ornamental plants are a common source of mold spores in buildings.  The leaves fall from the plant into the container where they are wet when the plant is watered, often resulting in heavy spore production.   In the outdoors, mold spores are formed in large numbers on all types of dead organic material, are spread through the air, and often enter homes through open doors or windows.  These are often the same species of molds that are found indoors.

Mold on wood wall (Photo: PDIC Database)

Molds can and do cause various kinds of damage in buildings, but for the most part the damage is cosmetic, such as stains on walls, floors, ceilings and furnishings.  

Heavy mold on wall (Photo: PDIC Database)

Molds also can produce unpleasant odors.  Most mold fungi are unable to decompose wood, thus do not cause structural damage.  Any structural damage in buildings where mold is present usually is caused by water per se rather than molds. At times, the presence of surface molds can indicate that wood decay fungi are also present. These fungi are capable of decomposing wood and, like mold fungi, are active at high levels of moisture. Often there is no visual evidence of wood decay fungi on the surface of the wood, but the presence of surface molds indicates conditions that could lead to damage by wood decay fungi.

White mold on window sill (Photo: PDIC Database)

Most people are concerned about the health risks associated with household molds, and in some cases these concerns are valid.  Many people have allergenic reactions such as headache, sneezing, runny nose, skin rash, etc. to high concentrations of certain mold spores. Other individuals can be in the same room at the same time and have no reaction.   Some mold fungi produce toxic compounds and exposure to these can pose more serious health hazards, especially in individuals that have a compromised immune system such as from radiation treatment or chemotherapy.  Unfortunately there have been numerous reports in the media about the danger to human health from the so-called black toxic mold Stachybotrys chartarum, especially in buildings that have been flooded.  More recent research has shown that this fungus is not nearly as dangerous as once reported.

Mold on windowsill (Photo: PDIC Database)

Most mold problems in homes can be detected visually when the colonies develop on walls, ceilings and floors.  However, molds can develop in other areas where they may not be easily seen, such within walls, attics, crawl spaces, within HVAC ducts, or inside cabinets, especially cabinets under sinks where water leaks often occur.  The pile of carpets made with artificial fibers is usually resistant to mold growth; however, the backing of most of these carpets support mold growth if sufficient moisture is present.  One should automatically assume that when water leaks occur, there is an excellent chance of mold development until the water problem is corrected and the wetted material has dried out. 

Mold on wooden chair. This chair was packed for shipping before it dried completely, resulting in mold development in transport.  (Photo: PDIC Database)

The identification of household mold fungi, especially to the level of species, is often difficult and time consuming.  One method of sampling mold spores in buildings uses an air sampler that draws air over a microscope slide coated with a sticky substance onto which the spores adhere.  The slide is then examined under the microscope and the spores identified, usually to the level of genus or groups of genera.  It is not possible to identify the spores to the level of species using this method.  This is understandable when one considers that the genera Aspergillus and Penicillium, some species of which are common household molds, each contain more that 200 known species.  This method does, however, give a relative measure of the number of spores in the air at the time of sampling.

Colony of Penicillium (Photo: PDIC Database)

Another method uses culture plates containing a general purpose growth medium that can be purchased at stores such as Lowes or Home Depot or your local hardware store.  The lid from the plate is removed and the bottom containing the medium is placed on the floor.  Plates are usually placed in different rooms in the house, and in the attic and crawl space.  Since the spores fall onto the plates by gravity, it is helpful to do the sampling when very little activity is going on in the house and the ventilation fan is off.  After a prescribed period of time, perhaps one hour, the lids are replaced and the plates are left at room temperature for several days.  

Culture plate submitted by homeowner (Photo: PDIC Database)

At this time the mold colonies are beginning to develop, usually one colony develops from one spore.  The molds on the plate can be identified by sending the plates to private laboratories as indicated by the instructions in the package.

The Plant Disease and Insect Clinic also provides mold identification services. To submit a sample, collect mold by swiping a clean cotton swab through the  colony, and immediately placing the swab in a clean plastic bag and sealing it.  A separate sample should be taken for each type of mold colony seen in each room or from different rooms.  Submit the samples to the Plant Disease and Insect Clinic, where they will be cultured on growth media.  Identification will be made at least to the level of genus and to the level of species for the more common molds.  This method gives only a relative indication on how much mold is present. We also accept culture plates collected as described above. 

Several mold/ yeast varieties identified by the PDIC. Culture plates were streaked with a cotton swab and sealed.  After several days, the diagnostician examines the colony types for identification. (Photo: PDIC Database)

Most mold can be removed by wiping the moldy area with a cloth wet in a ten percent solution of common household bleach.  Finished surfaces or fabrics should be tested to see if the bleach will harm the surface.  If the mold growth is heavy, a face mask should be worn while cleaning.  Household bleaches have little residual activity, and it may be necessary to clean again until the substrate is too dry to support mold growth.  If the mold is covering a large area which requires removal of water-damaged floor, walls or ceiling, the room should be sealed by closing the door or sealing the opening with plastic while the renovation is being done so that the spores do not spread to the rest of the house.  There are a large number of private mold remediation companies, which can safely remove mold from buildings.

Mold growing on a fabric cap (Photo: PDIC Database)

The most important thing that the public can do about molds is to prevent water problems or quickly correct them when they are found.