Friday, June 5, 2009
We have had our first report of the year of downy mildew on cucumber in NC. The outbreak is in Franklin Co, NC, located in the northeast of Wake County (Raleigh).
This disease favors the cool, wet weather we have been experiencing here in NC. Though we have not had any confirmed cases in WNC, it would be worth your time to scout your fields. Also, follow the Cucurbit Downy Mildew Forecasts for updated information.
Downy mildew of cucurbits is a disease that is very difficult to control once it becomes established in your field and can spread very quickly (see pictures below- Top picture = healthy cucumber field, Bottom picture = same field a few weeks later). Research has shown that by delaying fungicide sprays to cucumber just one week after the disease has become established in the field, yield losses of greater than 50% can occur (especially in cool, wet weather).
The disease spreads via wind currents from downy mildew infected areas. It can also be spread throughout a field by water splashes.
Symptoms on cucumber, squash and melon begin as yellow spots on the leaf. The spots are bound by the leaf veins and, as a result, appear angular. On the underside of the leaf, the spots may appear water-soaked, especially in early morning when moisture is still present in the plant canopy. You may also see a light gray to dark almost black, downy growth. Those are the spores (zoosporangia) of the pathogen, Pseudoperonospora cubensis. Eventually these lesions, and then the entire leaf, will become necrotic and turn brown. The leaf may appear crispy, like it has been burned. Because of rapid onset and crispy leaves, this disease has been referred to as "wild fire:
The following products* have proven very effective for the control of downy mildew:
Presidio 4F (fluopicolide)
Ranman 400 CS (cyazofamid)- must use with non-ionic surfactant
Gavel 75DF (zoxamide + mancozeb)
Curzate 60DF (cymoxanil)
Chlorothalonil and mancozeb products can be sprayed as protectants, but when downy mildew is detected in a field growers need to take a more aggressive approach. *The label on some of these products require that they be tank mixed with a fungicide of a different mode of action. Chlorothalonil and mancozeb products make excellent mixing partners.
A sample spray program might look like this:
Curzate 60DF (3.2 oz/acre) + Manzate Pro-Stick 75DG (3 lb/acre) alternated with Previcur Flex 6F (1.2 pt/acre) + Manzate Pro-Stick 75DG (3 lb/acre)
Presidio 4F (3 fl oz/acre) + Bravo Weather Stik 6SC (2 pt/acre) alternated with Gavel 75DF (1.5 lb/acre)
These treatments can be rotated on a 7-day schedule when downy mildew pressure is great.
To read more about this disease visit the NCSU Factsheet.
In addition, look for updates in the latest NCSU Pest News.
***As always, follow the product label for instructions!***