Diseases

Poinsettia - Powdery mildew

Powdery mildew (Oidium species) was first seen in US greenhouses in the 1990s and has since occurred sporadically. White colonies occur on both leaves and flower bracts, reducing aesthetic value.  It has no known hosts other than the poinsettia so will not survive in northern climates unless it is carried over on poinsettias in greenhouses. It is moved from greenhouse to greenhouse on infected cuttings or plants. The high temperatures of the summer months limit its growth.  Powdery mildews, unlike most other fungal diseases, do not need free water to germinate and infect.

Pepper - Tomato Spotted Wilt Virus (TSWV)

TSWV (Tomato Spotted Wilt Virus) and INSV (impatiens necrotic spot virus) are two members of the Tospoviruses which are vectored by Western flower thrips (Frankliniella occidentalis). These viruses have an extremely wide host range (more than 600 species of plants are susceptible). There is no cure or chemical treatment for plant viruses. Discard affected plants, eradicate weeds that may be hosts, and control thrips populations. Inspect plant material at arrival or quarantine new shipments. Do not grow vegetable transplants and ornamental bedding plants in the same greenhouse.

Basil - Pseudomonas

Pseudomonas cichorii causes a bacterial leaf spot on basil. The bacterium has a wide host range, but is not known to exhibit host specificity. P. cichorii is most important in chrysanthemum, geranium, and many other ornamental and foliage plants. Like Pseudomonas syringae, P. cichorii can be present on asymptomatic plants (epiphytic populations) allowing long distance distribution of the bacterium via propagative material.  Local spread within a crop is by rain or irrigation.

Root Diseases of Garden Mums

Monitor root health and test growing media of garden mums. Take random samples of plants out of their pots and examine roots carefully. The roots tell a lot about overall plant’s health, often before the top growth shows symptoms. Signs of poor root health are blackened or rotted roots or the lack of roots, especially young feeder roots. Diseased roots will not take up fertilizer.

Impatiens Downy Mildew Update

Bob Luczai, Mass Flower Growers Association reported Impatiens downy mildew last week in Dover, Wellesley and Sherborn, MA in landscapes, the first reports in MA in 2014 that I am aware of.

Growers and retailers have been asking if the disease is still a problem and should they begin to grow garden impatiens next spring.  Customers continue to want to plant garden impatiens, so demand (and likely a little pressure) continues. Because of this, some growers started producing garden impatiens this season (2014) when they did not produce them last year (2013).

Greenhouse Cucumber - Botrytis Stem Canker

Closely spaced plants, overcast cloudy weather, and flower drop from overhead hanging baskets encourage Botrytis blight on many greenhouse crops. The fuzzy grayish-brown spores, are easily spread on air currents and by water splash.

This pathogen is ubiquitous in the environment, has an extremely wide host range, and prefers to attack senescent and/or injured tissue. Botrytis can be controlled by management of environmental conditions, sound cultural practices, and fungicide applications.

Downy Mildew of Coleus

Downy mildew of Coleus, Peronospora lamii, was first detected in New York and Louisiana in 2005; by 2006 it was present throughout the United States. Downy mildew is closely related to Phytophthora and Pythium. Symptoms include brown, irregular lesions on leaves and leaf drop. Because the lesions are irregular, infection can cause the leaves to twist and distort before dropping. In cool, wet, humid conditions sporangia may be visible as a downy gray to purplish growth on leaf undersides.

Black root rot (Thielaviopsis basicola)

Black root rot (Thielaviopsis basicola) was diagnosed on calibrachoa. Roots and lower stems may be shriveled, brown to black. The black roots can be seen by washing roots free of growing media and then viewing them carefully with a hand lens or microscope.  This fungus thrives in cool weather and at alkaline media. As temperatures warm up injury to the root system may slow down.  However, infected plants and infested growing media should be discarded because fungicides will not eradicate the fungus, and you don't want to pass the problem to your customers.

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