Recognize late blight symptoms and know what works for disease control

Dry weather across Michigan does not favor late blight for tomatoes and potatoes.

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. Late blight is troublesome this season for tomato and potato growers in the northeast United States that have had an especially wet spring and summer. Michigan’s dry weather does not favor this disease.

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, or 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.

late blight stem
Dark, late blight lesions on stem of tomato.

late blight leaves
Late blight lesions on tomato leaves.

Between cropping seasons, the fungus survives on volunteer and abandoned potatoes in cull piles. Control measures include eliminating all potato and 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.

There are several highly effective fungicide sprays that can be used to protect tomato plants from late blight. Homeowners and organic growers have limited tools available and are not always able to hold the disease once it started. Commercial growers who use fungicides as part of their overall IPM program will be protected as many of the fungicides used for Alternaria, Septoria, and Anthracnose (e.g., Bravo, Quadris, Pristine, Tanos and Manzate) also provide some protection against late blight, especially early in the season. If late blight disease is found, the addition of late blight specific fungicides to the spray program is very helpful in controlling the disease. Find fungicides registered for late blight on tomatoes.

For the last 11 years, the Hausbeck lab has conducted late blight trials on tomatoes. 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 has been documented in a particular growing region, 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 (1 to 2 percent) 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 (percent). Yields were taken from the inner 6 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 (see 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. 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
Click here for larger version.
late blight figure 

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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