April Showers Bring May Flowers... and DISEASES!

Last week, we told you about diseases to watch for in your home vegetable garden. In this post, we will tell you about some diseases to watch for in the home landscape. We have already seen these diseases active around campus and you can probably find them hanging out in your own back yards. 

Septoria Leaf Spot on Black Eyed Susan 

Last week we wrote about Septoria leaf spot on tomato. A different species of Septoria, Septoria rudbeckiae, attacks black eyed Susan and other members of the genera Rudbeckia and Ratibida. This species of Septoria will not cause disease on tomatoes! Symptoms on Rudbeckia begin as small, dark-brown lesions that enlarge as the disease progresses. As the lesions age, small black dots (fruiting structure of the fungus) can be seen in the lesions. Spores are released in late spring and early summer and are spread by splashing water. Lower leaves are usually infected first. To manage this disease, avoid the use of overhead watering and practice good sanitation. Remove fallen leaves at the end of the season to reduce inoculum levels for the following year. Improve air circulation in your beds by spacing plants out. Chemical control can be achieved through the use of preventative fungicides. Products containing chlorothalonil or mancozeb are recommended. 

Septoria Leaf Spot on Rudbeckia (Photo: M.J. Munster)

Hollyhock Rust 

Hollyhock rust is caused by the fungus Puccinia malvacearum. This very destructive disease often limits the use of hollyhocks in the Southern landscape. Symptoms of rust infection begin as light yellow to orange spots on the upper leaf surface. Brown pustules develop on the underside of the leaves and erupt to reveal bright orange rust spores. These spores are windblown to healthy leaves and infection begins again. Disease increases rapidly because hollyhock rust can infect over and over again. The plants become very unsightly as the infected leaves start to die and fall off. Typically, rusts are highly host-specific. This particular rust infects hollyhocks and other members of the mallow family. Control rust by limiting leaf wetness. Avoid overhead watering and space plants to improve air circulation and promote leaf drying. Rust can overwinter in plant debris so sanitation is important. Remove infected leaves in early spring and remove infected stalks and lower leaves at the end of the season. Plant debris should be buried or burned. Removing nearby susceptible plants, like rose of Sharon and mallow weeds, is important to prevent new sources of inoculum. Chemical control can be achieved through the use of products containing chlorothalonil or myclobutanil. Some seed catalogues list rust-resistant hollyhocks but we do not know of any trial results in this area. Ultimately, you may be forced to forgo planting hollyhocks. 

Hollyhock Rust (Photo: M.J. Munster)

Hollyhock Rust (Photo: M.J. Munster)

Powdery Mildews on Euonymous, Gerbera Daisy, Dogwood and Rose 

Right now, several powdery mildews are active in the landscape. Be on the look-out for powdery mildew on euonymous, gerbera daisies, dogwoods, and roses. Control of powdery mildews can be achieved through sanitation and pruning during dormancy. Take care not to over-fertilize since succulent leaves are more susceptible to infection. The best control is host resistance so try to plant resistant varieties whenever possible. Several active ingredients provide effective chemical control of powdery mildews, but control requires applications on continuous 7-14 day intervals. Repeated sprays with the same fungicide can select for fungicide-resistant strains of powdery mildew. For more detailed information about powdery mildews, click here to read our earlier blog post. 

Powdery Mildew on Euonymous (Photo: M.J. Munster)

Powdery Mildew on Gerbera Daisy (Photo: M.J. Munster)

Powdery Mildew on Rose (Photo: M.J. Munster)

Powdery Mildew on Dogwood (Photo: B.B. Shew)

Azalea and Camellia Leaf Gall 

Azalea leaf gall is caused by the fungus Exobasidium vacinii. This disease is common and widespread in the spring and often attacks new leaves and flower buds. On azalea, infected leaves become thickened, curled, fleshy and pale green to white, forming galls. Infected azalea flowers are usually pale pink. In later stages of disease, white powdery spores cover the surface of infected tissue. A different species of Exobasidium is very common on new growth of sasanqua camellia. On camellia, entire leaves are swollen and distorted and extensive swelling causes the leaf epidermis to split, revealing the white spores beneath. Spores of Exobasidium are windblown or splashed to healthy leaves or flower buds. New infections are not evident until gall-like swellings form the following year. Eventually affected leaves and flowers turn brown, become hard, and fall to the ground. This disease is favored by cool, wet weather. Although unsightly, infected plants are not usually seriously damaged. This disease can be problematic in a greenhouse setting where humid conditions promote sporulation and disease spread. The best control is sanitation. Pick or prune off diseased tissue as soon as swelling starts. Be sure to remove infected tissue before the fungus starts producing spores to prevent new infections next year. Burn or bury the swollen tissue. Often disease is not severe enough to warrant chemical control. When possible, try planting resistant varieties. Susceptible varieties of azalea include White Gumpo, Rosebud, Mother’s Day, and China Seas. Resistant varieties include Amonena, Gloria, Coral Bells, Glacier, Formosa, and Aphrodite. Exobasidium vacinii can also infect blueberries and is responsible for the “green spot” symptom on fruit and leaves. See Bill Cline’s post on Exobasidium vacinii infections of blueberry for more information.

Leaf Gall on Azalea (Photo: H.D. Shew)

Leaf Gall on Camellia (Photo: H.D. Shew)

Leaf Gall Symptoms on Camellia (Photo: H.D. Shew)

Leaf Gall Symptoms on Camellia (Photo: B.B. Shew)

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

My plants have a little sugar coating :) Powdery Mildew Spotted in the Landscape

Powdery Mildew on Crape Myrtle
(Photo: H.D. Shew)

Powdery mildew has started to show up on many landscape plants. Powdery mildew is easily diagnosed on most plants by the characteristic coating on leaves that gives this disease its name. The “powder” will not rub off – it is actually the mycelium and spores of the disease-causing fungus. 

Powdery mildews frequently infect dogwoods, crape myrtles, roses, lilacs, tulip-trees, oaks, and numerous other shrubs and perennial plants.  In addition to the powdered appearance, typical symptoms include discolored and distorted leaves, shoots, and flowers. Although powdery mildews rarely cause serious damage to most hosts, severe cases can result in defoliation or poor growth. 

PM on fruit
(Photo: H.D. Shew)

PM on flower stem
(Photo: H.D. Shew)

While most powdery mildews look the same to the naked eye, hundreds of different species of fungi cause powdery mildews. Most are specific to a particular host. Under the microscope, different groups of powdery mildews can be identified by their highly distinctive cleistothecia (specialized spore-bearing structures).

PM micrograph
Note hooked appendages
(Photo: H.D. Shew)

PM micrograph
Note branched appendages
(Photo: H.D. Shew)

In the spring, cleistothecia from previous infections rupture and release wind-blown spores.  These spores land on susceptible plant tissue and begin to grow on the surface and into the upper layer on the leaf.   The fungus produces columns of asexual spores that are capable of causing new cycles of infection.  Late in the season, the cleistothecia appear as tiny dark specks embedded in the mildew.  These contain the sexual spores of the fungus and will overwinter to start new infections in the spring.

Black cleistothecia embedded in mycelial mat
on leaf surface (Photo: H.D. Shew)

Powdery mildew is most common in cool or warm and humid weather conditions, but it may occur and cause severe damage in warm and dry climates. Temperatures between 70-80 degrees F and a relative humidity between 85%-100% favor infection. Powdery mildew pathogens are some of the only fungal organisms that can germinate and infect in the absence of free water.  These fungi require high humidity but are not dependent on wet leaves for infection and spread.  Excess leaf wetness actually inhibits spore germination. 

Management:
Cultural Control:

• Purchase mildew resistant varieties
• Reduce humidity: Increase air circulation and light penetration by thinning and pruning plants to reduce overcrowding
• Avoid planting susceptible plants in shaded areas
• Sanitation: Prune out dead and diseased tissue and rake and remove any fallen tissue to reduce the chances of the fungus surviving until the following season
• Overhead watering of the leaves: Water inhibits spore germination for most powdery mildews, but be careful because excess leaf wetness can increase chances of other foliar diseases.  
• Avoid practices that stimulate succulent growth: Powdery mildews are obligate parasites that prefer fresh, succulent plant growth. Applying nitrogen fertilizer, pruning heavily, and watering excessively are not recommended because they promote succulent growth.  

Powdery mildew on dogwood (Photo: H.D. Shew)

Chemical Control: 

Chemical control is often not necessary in the landscape because this disease rarely kills the plant.  Spraying trees such as dogwoods and crape myrtles can be impractical. 

Fungicides may be used to control powdery mildew on highly susceptible varieties of roses and other plants that suffer severe symptoms and damage.

See Rose Diseases and Their Control in the Home Garden
Rose Diseases Link