So far it's only in the Northeast, but this scourge affecting potatoes and tomatoes easily could move in any direction. In addition, basil downy mildew is affecting plants in the Northeast.
Normally a late blight, it is unusual, never known to occur this early and this widespread in the U.S., says Meg McGrath, associate professor of plant pathology and plant microbe biology.
One of the most visible early symptoms is brown spots (lesions) on the stem. They being small and firm, then quickly enlarge, with white fungal growth developing under moist conditions that leads to soft rot collapsing the stem.
Other classic symptoms:
- large (at least nickel-sized) olive-green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the border of the spot is yellow or has a water-soaked appearance. Spots begin tiny, irregularly shaped and brown. Firm, brown spots develop on tomato fruit.
It can't be stressed enough that a rapid response to protect garden-grown tomatoes and potato plants from this fungus must be made. Also be sure that the plants don't become a source for spores that could infect commercial farms, as late blight spores are easily dispersed by wind.
McGrath also recommends that gardners:
- Examine their tomato and potato plants thoroughly at least once a week for signs of late blight;
- Spray fungicides preventively and regularly, and/or
- Be prepared to destroy plants when late blight starts to become severe.
Start spraying with fungicides before you see the symptoms, and continue regular spraying thereafter. "Use a product that contains chlorathalonil as copper is not very effective on late blight," says McGrath.
Petunias, which are closely related to tomatoes and potatoes, can also be infected and will show similar symptoms.
Late blight is very destructive. Uncontrolled, it will kill plants faster than any other disease. And it affects tomato fruit -- especially green ones. Even with fungicide applied every week, there is no guarantee of success, especially if the rainy weather continues. McGrath recommends that gardeners consider growing more of other vegetables this year.
One source of late blight in New York has been traced to tomato plants imported to garden centers from production facilities in the south. If tomatoes were started from seed by a gardener or a farmer in the Northeast, plants are unlikely to be infected, at least initially, she said. If plants were purchased at a garden center and they show signs of late blight, McGrath recommends contacting a local office of Cornell Cooperative Extension or Cornell's Plant Disease Diagnostic Clinic to get confirmation and tell them where you purchased the plants.
Another source of infection is the weed hairy nightshade that acts as a host. Hairy nightside typically grows alongside potatoes and on the margins of farms. It is also a poisonous medical plant.
You can tell is the weed is infected if it shows up with "speckled and suspicious dark and oily spots."
The State of Maine discovered that 55% of fields assessed in 2007 contained the plant, a secondary source of the destructive disease.
The weed can grow anywhere. In fact, after reading this, I noticed some plants growing nearby that I immediately eradicated. But, as I found out with fungus and insects affecting citrus, by the time see them, it's often too late.
Surely the same holds true for the late blight and it's host, hairy nightshade.
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