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