Monday, July 20, 2009

Cucurbit Downy Mildew- Moderate Risk for WNC

Downy mildew (Pseudoperonospora cubensis) has been threatening cucurbits in NC since early June. There have been no reports of downy mildew on cucurbits in WNC and we have been lucky to have been considered at LOW RISK for chance of infection, until now...

The Cucurbit Downy Mildew Forecasts for today and tomorrow show WNC as having a MODERATE RISK for infection. A MODERATE RISK means that there is a 40-60% chance for cucurbits growing in areas along the course of the trajectory (or trajectories, in the case of multiple sources) to be infected by the pathogen and develop downy mildew. To read about how this is calculated refer to the Downy Mildew Forecasting site.

The trajectories that may effect WNC can be seen below.

As you can see there is the potential of spore deposition in WNC from multiple sources. If the weather continues to include cloudy skies and the potential of rain (to wash the spores from the sky) growers should begin to take action to protect their cucurbit crops.

The following products* have proven very effective for the conventional control of downy mildew of cucurbits:

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.

ORGANIC GROWERS have a chance to combat downy mildew if they begin applications now before the onset of symptoms. If you wait until infection has established in your planting, spraying will not be effective. The same is true in conventional systems. Some OMRI-listed products that are labeled for the management of downy mildew in cucurbits include: copper, neem, biofungicides (ex. Serenade), peroxides (ex. OxiDate), and bicarbonates (ex. Kaligreen).

I have had a lot of experience with this disease, as my master's thesis was focused on the control of downy mildew on cucurbits. Our research showed that most organic fungicides were NOT effective at controlling downy mildew on cucumber. Once the disease develops and conditions are favorable, it is nearly impossible to control it.

Your best option as an organic grower is to use a copper product. Spray early in the morning to avoid phytotoxicity that may develop in the heat of the afternoon. Spraying copper prior to disease development or at very early onset (very few, mild symptoms), may help suppress the disease, but will not offer 100% control under favorable conditions (cool, wet and humid weather).

If you suspect that you have downy mildew on your cucurbits, please contact your local cooperative extension office. It is very important that these cases are documented and reported to the forecasting site in a timely manner.

To read more about this disease or to view some pictures, visit the blog post from 5 June and the NCSU Factsheet.

In addition, look for updates in the latest NCSU Pest News.

***As always, follow the product label for instructions!***

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