Preventing late blight on tomatoes is easier than you think

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Late blight is a disease that most commonly affects potatoes, but can affect tomatoes when the weather is cool, rainy and humid. The pathogen is called Phytophthora infestans and is well known to potato growers in the Northeast who suffered severe losses from this disease in 2008. Late blight was troublesome this season for tomato and potato growers in regions that had an especially wet spring followed by an unusually cool and wet summer. Late blight symptoms include blighting on all above ground parts of the tomato plant. Lesions on leaves often appear dark and oily with production of sporangia (a.k.a., seeds of the pathogen) occurring on the undersides of the leaves resulting in a whitish-purplish appearance especially when conditions are wet and humid. These sporangia can be carried long distances from diseased plants to nearby healthy plants via wind currents and storm fronts. Blackened lesions on the stems also occur and are typical of late blight disease. Late blight affects green and ripe tomato fruit. The blighting on the fruit appears as dark, greasy areas that enlarge rapidly, encompassing the entire fruit. During wet and humid conditions, white masses (sporangia and threads) of the late blight pathogen can be seen on the diseased leaves and fruit.

Between cropping seasons, the fungus survives on volunteer and abandoned potatoes in cull piles. Control measures include eliminating all potato and/or tomato cull piles and destroying volunteer potato plants that grow from overwintered tubers. Infected potato plants established from diseased seed potatoes are another source of late blight. Most tomato varieties are susceptible to late blight. ‘Mountain Magic’ is a late blight resistant variety recently developed at North Carolina State University and is in low supply at present.

There are several highly effective fungicide sprays that can be used to protect tomato plants from late blight. Homeowners and organic growers had limited tools available and were not always able to hold the disease once it started. Conventional growers who use fungicides as part of their overall IPM program fared better as many of the fungicides used for Alternaria, Septoria, and Anthracnose (i.e., Bravo, Quadris, Pristine, Tanos, and Manzate) also provided some protection against late blight, especially early in the season. As the late blight disease increased in growing regions, and the weather continued to be cool and rainy, the addition of late blight specific fungicides to the spray program were helpful in controlling the disease, yet were not always 100 percent effective.

For decades, late blight trials on potatoes have been conducted at the Muck Soils Research Farm of Michigan State University. This research farm has been the focus of potato late blight trials conducted annually by Dr. William Kirk (Plant Pathology) and Dr. David Douches (Crop and Soil Sciences), which are inoculated each year near the end of the growing season to assess fungicide efficacy and genetic resistance. For the last 11 years, my lab has conducted late blight trials on tomato after the potato researchers introduced the late blight pathogen to their nearby potato trials. This year, two tomato late blight fungicide trials were conducted at the Muck Soils Research Farm and one trial was established at the Plant Pathology Farm. The first late blight symptoms in our research plots were observed on August 24 and resulted from the pathogen’s sporangia being carried on air currents from diseased plants in other regions.

One of our tomato late blight trials involved a comparison of single products (Table 1). Although growers should always alternate among fungicides based on their mode of action, it is valuable to know which products are most effective and therefore should be used during cool and rainy weather. When late blight threatens, it is best that late blight fungicides be applied preventively prior to the development of symptoms. In our research trial, fungicide sprays were applied after a trace (one to two perceont) of disease was detected in our research plot (August 24), using a CO2 backpack sprayer equipped with three XR8003 nozzles. The sprays were reapplied every seven days. The sprayer was operated at 50 psi at the boom and was calibrated to deliver 50 gal/A. Plots were evaluated after four weeks for foliar infection (percentage). Yields were taken from the inner six feet of row on September 22. Data were analyzed using Sigma Stat version 3.1 (Systat Software Inc.) and treatments were compared using the Fisher LSD multiple comparison test.

Table 1. Fungicide products tested for the control of tomato late blight, MSU Plant Pathology Farm.

Active ingredient
Registered for tomatoes
Bravo Weather Stik
Manzate Pro Stik
postassium phosphate
yes, Phytophthora spp.
Previcur Flex
Reason 500
Revus 2.08SC
Ridomil Gold Bravo
Ridomil Gold MZ

Plants that were not protected with fungicide were completely defoliated by the late blight pathogen. The fruit from the unprotected plants also became diseased and 84 percent showed blight (Figure 1). Most of the fungicides we included in this trial helped to limit the advance of late blight, but did not fully eradicate it. Reason and the new product Revus were the stand-out treatments of all fungicides tested in protecting both the foliage and fruit from disease. Using these new products, combined with other effective fungicides, can prevent the development of late blight. A number of other treatments were also impressive and included Pristine, Ranman, Ridomil Gold Bravo, Ridomil Gold MZ, Bravo Weather Stik, and Presidio. Keep in mind that other researchers have noted that this late blight strain is Ridomil-resistant so in the Ridomil Gold Bravo and Ridomil Gold MZ treatments, the Bravo and Mancozeb components, respectively, of those products may be the primary contributors to the late blight control. It is important to note that applications of Previcur Flex did not adequately protect the tomato foliage (35 percent infection) or the fruit (48 percent of the yield infected) from late blight. Similarly, Phostrol applications were not very helpful in limiting disease.

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Figure 1. Tomato late blight trial conducted at MSU Plant Pathology Farm.
Late blight on tomato graph

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