UNIVERSITY PARK, Pa. — Over the past few days, there has been an increasing number of confirmed reports and suspected outbreaks of late blight. These reports are from Erie, Mercer, Indiana, Centre, and Clinton counties, and are primarily in tomatoes in both commercial production and home gardens, especially where there have been limited or no fungicide applied.
As we head toward fall, with cooler night temperatures and longer dew periods, conditions are going to become increasingly favorable for late blight. The preferred temperatures range is from 50 to 75 degrees. Keep in mind that even if the temperatures reach into the upper 80s and conditions are sunny during the day, the disease will continue to progress under cooler night temperatures.
Thoroughly scout fields, especially higher risk areas such as lower-lying areas, areas that are more shaded, or where late blight has been a problem in the past.
The symptoms include pale green or water-soaked and gray in color on the leaves, petioles and stems. Under humid conditions, white sporulation (fuzzy growth) can develop, especially on the underside of the leaves, although it can also develop on the upper surface under favorable conditions. When released, these spores can spread the pathogen to nearby plants. When dried out, the lesions appear necrotic and brown to black in color.
Several conventional fungicides are effective for managing late blight if managed preventatively. Initiating a fungicide program after symptoms are observed can be challenging when conditions are favorable. However, on tomato, chlorothalonil can even be effective if applied on a weekly preventative schedule, and good coverage is obtained.
Late blight specific fungicides would include products such as but not limited to, Previcur Flex (FRAC 28), Ranman (21), Zampro (45+40) or Orondis Opti (U15+M5).
These products should be tank-mixed with a protectant for fungicide resistance management and alternated/rotated between different FRAC codes. For organic growers, copper-based programs tend to be most effective. Another possible option would be to alternate between Regalia and Actinovate both tank-mixed with a copper-based fungicide. These products are most effective when applied preventatively and regularly when conditions favor the disease.
Good spray coverage is essential. Remember that late blight is a community disease. If you suspect late blight, contact your local Penn State Extension office, the Penn State Plant Disease Clinic or me at email@example.com or 814-865-7328 for confirmation. Updates via audio-messages will be left on the 800-PENN-IPM hotline as well as posted at the produce auction information displays. For the most current map of confirmed late blight outbreaks, visit USABlight.org.
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