Jul 5, 2018
Downy mildew, late blight seen in mid-Atlantic region

Downy mildew has most recently been confirmed on cucumbers in southern New Jersey and Maryland. Sources of the disease are continuing to build in the southeastern U.S., but the current weather patterns are favoring a local rather than long-range spread of the disease to the mid-Atlantic region. Late blight has been confirmed on tomatoes in York County and on potatoes and tomatoes in southern Lancaster County. Samples collected from York County have been determined to be caused by Phytophthora infestans genotype US23. This is the same genotype that has been predominant on both tomatoes and potatoes across the region for the past four years.

Monday, July 2, downy mildew was confirmed in a commercial cucumber in southern New Jersey just south of Philadelphia. This is the second report in the region this season. Thanks to a good scouting and fungicide program in commercial pickling cucumber fields in Maryland there have been no new reports since mid-June. Current disease forecasting puts parts of eastern Pennsylvania at low risk for potential spread from these sources over the next several days. Sources of the disease are continuing to build in the southeastern US, but the current weather patterns are favoring more local spread rather than a long-range spread of the disease to the mid-Atlantic region.

Purple spores of the pathogen on the lower leaf surface of a cucumber. Photo: Beth Gugino, Penn State

Fungicides are an important tool for effective control. At this time, protectant fungicides are recommended for growers in the central and western parts of Pennsylvania, if not already being used. Conventional growers in eastern Pennsylvania closer to the sources in Jersey and Maryland may want to consider including a downy mildew specific fungicide in their spray program especially as we break out of this hot weather cycle towards the end of the week. Organic management of downy mildew is more challenging. Copper-based products are still the primary tool but can also cause phytotoxicity on some cucurbit crops. Other organic options include the microbial biopesticides Actinovate, Double Nickel 55, Serenade, and Sonata as well as the biochemical biopesticides MilStop, Organocide, Oxidate, Regalia, Sporatec, and Trilogy. Zonix is an OMRI-approved rhamnolipid surfactant which has shown some efficacy when included in a regular spray program. It is recommended that all organic cucumber crops be under some type of protectant spray program at this time.

There are more product options for non-organic production including Orondis Ultra (U15 + 40), Orondis Opti (U15 + M5), and Ranman 400SC (21) which can be rotated with Previcur Flex 6F (28), Zampro 525SC (40 + 45), Zing! 4.9SC (M5+ 22). See the 2018 Mid-Atlantic Commercial Vegetable Production Recommendationsfor a complete list of recommended products. For resistance management, rotate between FRAC code groups and tank mix with a protectant fungicide. The latest resistance management recommendations can be found electronically. In fall 2017, Valent received a federal registration for Elumin (ethaboxam, FRAC 22) which is labeled for oomycete diseases on cucurbits, ginseng, pepper, eggplant, and tuberous and corm vegetables.

Since downy mildew disease forecasting is an important tool utilized by growers, extension educators, crop consultants, and other industry stakeholders, confirming reports especially early in the season and on different cucurbit crops is important at both a local and regional level. If you suspect downy mildew on your farm, please let me know either by email at  or by phone at 814-865-7328 or contact your local Extension Office. Check the CDM ipmPIPE website for the latest information about confirmed reports.

Late blight confirmed in south central Pennsylvania

Late last week late blight was confirmed in a 3A conventionally managed commercial tomato field in York County and in a small ¾ A potato field and adjacent tomato field in southern Lancaster County. Both growers are working to actively manage the disease. Samples collected from York County have been determined to be caused by Phytophthora infestans genotype US23. This is the same genotype or strain that had been predominant on both tomato and potato across the region for over the past four years. Samples from Lancaster County have been submitted for genotyping.

Characteristic irregular brown lesions caused by late blight on the upper tomato leaf surface. Photo: Cathy Thomas, Pennsylvania Dept. of Agriculture

Late blight white sporulation on the lower leaf surface. Photo: Cathy Thomas, Pennsylvania Dept. of Agriculture

Similar to cucurbit downy mildew, there are a number of conventional fungicides that are very effective for managing late blight if managed preventatively. On tomato, chlorothalonil can even be effective if applied on a weekly schedule preventatively and good coverage is obtained. Protectant fungicides are recommended at the very least and in York and Lancaster County, it is recommended that late blight specific fungicide be incorporated into fungicide spray programs especially as the temperatures cool off later in the week. Late blight will not progress when temperatures reach above 90°F however, keep in mind that the high temperature will not kill the pathogen, so the disease will continue to progress when the temperature drops in the evening and the leaves are wet as a result of dew.

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). See the Mid-Atlantic Vegetable Production Recommendations for additional recommendations. 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 disease. Good spray coverage is essential.

If you suspect late blight on your farm, please contact your local Penn State Extension Office or let Beth Gugino know via email at  or by phone at 814-865-7328. We are interested in collecting samples so we can better understand how the pathogen population is changing both within and across growing seasons. Also for the information regarding where the latest confirmed outbreaks have been reported and to receive email or text alerts about when late blight has been confirmed with a personally defined radius from your location visit the USA Blight website.

Additional updates

  • Basil downy mildew is being reported across the region including south-central Pennsylvania and southern New Jersey.
  • Seeing some issue with insect-transmitted viruses as tomato spotted wilt on tomatoes in high tunnels.
  • Blackleg continues to be a problem in potatoes in some parts of Pennsylvania.
  • High temperatures and the intense sun are causing sunscald on some crops as well as phytotoxicity from spray injury.

Beth Gugino, Penn State University

Photo at top: Angular lesions caused by downy mildew on the upper surface of a cucumber leaf. Photo: Beth Gugino, Penn State

 





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