Late Blight in Southern Maryland

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Late blight on tomato occurred early in 2009 throughout the East Coast. Because the disease arrived early and was disseminated widely on infected transplants, growers had to apply fungicides season-long to maintain crop health. Unfortunately on May 7, 2010 a greenhouse outbreak of late blight on tomato was found in St. Mary’s County, MD. The grower destroyed the tomatoes in his greenhouse, but kept some high tunnel production, where the disease is being managed with fungicide applications. We do not know how late blight became established or whether it will spread quickly. The surrounding fields have been scouted extensively and no additional infection has been found. The disease is favored by cool wet weather, which often occurs in spring. Therefore, tomato growers should scout their crop rigorously and apply fungicides as described below.

Late blight is caused by the fungus-like organism Phytophthora infestans, which is an obligate parasite. The organism prefers cool moist weather and cannot overwinter in Delaware or Maryland (outside a living host) because only one mating type is known to occur in these states. (If the other mating type is introduced, the pathogen would be able to form resistant oospores.) In the absence of resistant oospores, the late blight fungus overwinters in infected potato tubers or is introduced into an area on wind, or infected plants. We don’t believe that the pathogen overwintered in St. Mary’s County, however we don’t know how it was introduced. The grower did not have any live plants that could have allowed late blight to overwinter.

Phytophthora infestans can infect leaves, stems and fruit of tomatoes (Figures 1-3). Lesions on leaves are large and dark brown. Purplish or whitish growth on the lower surface of the lesions occurs under humid conditions. Fruit lesions initially appear water soaked, turn dark brown, expand rapidly, and are shiny.

There are several fungicides available that will help reduce disease spread. No fungicide however, will eradicate the disease. To be most effective, fungicides should be applied prior to disease onset. For this reason, once plants reach a height of 6 inches, protectant fungicide should be applied every seven days. Chlorothalonil (Bravo), Gavel or mancozeb are good choices.

Once the disease is observed in the area, switch to a translaminar fungicide which can move into and through the leaves. Also, it is important to note that while the most common previously occurring P. infestans genotypes were resistant to Ridomil, the genotype that occurred on tomatoes in 2009 (called US22) is sensitive to Ridomil. In addition, there are several other fungicides listed in the Delaware Commercial Vegetable Production Recommendations, which can be used for late blight management.

Rotate between the following tank mixtures:

● Curzate–3.2-5.0 oz 60DF/A plus a protectant fungicide
● Forum–6.0 fl oz 4.18SC/A plus a protectant fungicide
● Presidio–3.0–4.0 floz 4SC/A plus a protectant fungicide
● Previcur Flex–1.5 pt 6F/A plus a protectant fungicide
● Ranman–2.1-2.75 fl oz 400SC/A plus a protectant fungicide
● Revus Top–5.5–7.0 floz 4.16SC/A plus a protectant fungicide
● Tanos–8.0 oz 50WG/A plus a protectant fungicide
● Reason –5.5 – 8.2 fl. oz plus a protectant fungicide



Late blight symptoms on leaves (Figure 1), stems (Figure 2) and fruit (Figure 3) of tomato (images courtesy of Dr. Meg McGrath, Cornell University).

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