Late Blight of tomato has been detected in Massachusetts for the first time this growing season. This disease is caused by the fungus-like organism Phytophthora infestans and can affect both tomatoes and potatoes. It should not be confused with Phytophthora blight (caused by P. capsici), which affects squash as well as tomatoes and peppers. Late blight has also been reported to affect petunias and nightshades.
Symptoms of late blight include dark, water-soaked lesions on leaves, often with fuzzy white sporulation on the underside. Lesions can also occur on stems. Sporangia (asexually produced spores) are dispersed by splashing water or by wind. Under optimum conditions, three to four days may elapse between infection and the appearance of symptoms.
Infected plants should be removed and disposed of. Do not compost infected material. Volunteer potato plants should be removed from fields or gardens when they emerge in spring as the pathogen may survive the winter in infected tubers. Control solanaceous weeds such as nightshade and henbane.
Fungicides may be used to protect remaining healthy plants from infection. Keep in mind that infected plants should still be removed when possible and that plants may be infected but not yet showing symptoms. Fungicides will not make existing infections disappear, but can protect uninfected plants from becoming infected. Please see UMass Extension fact sheet on solanaceous late blight (https://ag.umass.edu/vegetable/fact-sheets/solanaceous-late-blight) for more information. For information about fungicide options please see the New England Vegetable Management Guide (http://nevegetable.org)
It is always best to start with disease-free plants. Purchase certified seed potatoes; do not save seed from infected crops or plant tubers purchased in grocery stores. Start tomatoes from seed (P. infestans does not infect tomato seed) or buy seedlings that have been produced locally. Keep vegetables and ornamentals well separated in the greenhouse. Grow resistant or tolerant varieties: these include tomato cultivars ‘Iron Lady’, ‘Mountain Merit’, ‘Mountain Magic’, ‘Defiant PHR’, ‘Legend’, and ‘Matt’s Wild Cherry’, among others. Please see (http://vegetablemdonline.ppath.cornell.edu) for more information on resistant and tolerant varieties.
Farmers, greenhouse growers, and home gardeners should have possible late blight infections positively identified by Extension personnel. Please see UMass Plant Diagnostic Lab (http://ag.umass.edu/services/plant-diagnostics-laboratory) for information about submitting samples.
Report by Dr. Angela Madeiras, UMass Extension Plant Diagnostic Lab
Geoffrey Njue, UMass Extension