Vegetable Disease Updates for July 26, 2017

The rainy weather is not letting up and neither is downy mildew pressure.
Vegetable Disease Updates for July 26, 2017 - News

Updated: April 8, 2018

Vegetable Disease Updates for July 26, 2017

Map of cucurbit downy mildew currently reported along the east coast as of 26 July 2017 (cdm.ipmpipe.org).

Weather Favors Cucurbit Downy Mildew

Currently it has been confirmed in eleven counties in central and southern Pennsylvania with an unconfirmed report on cucumber in western PA. Cucumber continues to be the primary host however there are also several reports on cantaloupe across the region. The nearest reports on jack-lantern type pumpkin are in South Carolina and Mississippi.

With successive cucumber plantings, once harvest is complete, disk or burn down the crop debris to reduce the further movement of downy mildew inoculum into the younger plantings. As with late blight, once the plant tissue is dead, the pathogen is dead. Also maintain a regular fungicide spray program on the later plantings. Cucurbits are susceptible to downy mildew at any stage of growth.

Downy mildew can often be confused with the bacterial disease angular leaf spot (see comparison photos below). The symptoms are very similar; the lesions are initially water soaked in appearance before turning brown or straw-colored and are also vein limited. The lesions will often dry and drop out, leaving irregular shaped holes in the leaves. If you do not see downy mildew sporulation on the underside of the leaf in the field, place several symptomatic leaves in a sealed bag overnight and then check for purplish gray sporulation the next day.

For the latest information on outbreaks and to receive email or text alerts please visit the Cucurbit Downy Mildew Forecasting website. Updates will also be made to the 1-800-PENN-IPM hotline weekly or more frequently if needed to provide growers with information that can be used to help make timely management decisions. The forecasted risk maps are also based on knowing where there are downy mildew infected fields (sources of the pathogen) so it is important if you suspect downy mildew on your farm contact Beth Gugino by email at bkgugino@psu.edu or by phone at 814-865-7328 or contact your local Penn State Extension Office.


Cucumber leaf showing symptoms of both angular leaf spot and downy mildew. Downy mildew lesions have purplish gray sporulation on the underside of the leaves (upper right) while those infected with angular leaf spot do not (lower right). Photos: Beth K. Gugino, Penn State

Broken Record: Still No Reports Of Late Blight In Pennsylvania

To-date, there are no confirmed reports of late blight in Pennsylvania and not new reports across the region. Previous reports of late blight nearby include tomato in Ontario Canada and potato in Michigan, the eastern shore of Virginia and in Erie and Livingston Counties in New York.

Although there are currently no reports of late blight in Pennsylvania it is still important to scout higher risk areas of the field including low lying areas or shaded field edges and potato cull piles for symptomatic volunteers. If you suspect late blight on your farm, please contact your local Penn State Extension Office or let Beth Gugino know via email at bkgugino@psu.edu 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 USABlight.org.

Increased Reports Of Phytophthora Blight And Other Wet Weather Diseases

The wet weather has been very favorable for Phytophthora blight on both cucurbits and peppers. Initial symptoms of Phytophthora fruit rot are water soaked or depressed spots typically on the underside of the fruit where it is in contact with the soil. Symptoms can develop on the upper side of the fruit following rain or an irrigation event that splashes infested soil and spores up onto the fruit. Eventually the fruit will become covered with white sporangia and will rapidly collapse either in the field or shortly after harvest. The sporangia form when the soil is at field capacity and the infective zoospores that cause new infections are released when the soil is saturated which explains why the disease is most prevalent poorly drained soil and/or after significant rainfall. Young shoots and leaves will wilt and eventually collapse. The lower crown can turn tan to brown and develop a soft rot.

Managing soil moisture by avoiding planting in poorly drained soils or low lying areas, sub-soiling to break-up hard pans, raised beds and avoiding excessive irrigation. Do not irrigate from ponds or surface water sources that may contain water that drained from infested fields. Minimize splash dispersal of spores and surface water movement between rows or fields. In fields where Phytophthora is a problem minimize movement of people and equipment from infested to uninfested fields. Remove diseased plants and fruit from the field. This will reduce the spread of secondary inoculum. If sections of the field are very bad, consider disking these areas under to reduce pathogen inoculum and further spread in the field. Avoid culling infected fruit into production fields.

Under favorable conditions, fungicides will only suppress Phytophthora at best. In addition to the recommendations in the 2016/2017 Mid-Atlantic Commercial Vegetable Production Recommendations, new products registered for Phytophthora blight on cucurbits include Orondis Gold 200 tank mixed with Orondis Gold B which is oxathiapiprolin (FRAC U15) tank mixed with mefenoxam (FRAC 4), Zampro (ametoctradin + dimethomorph; FRAC 45 + 40) and Omega (fluazinam; FRAC 29).


Characteristic collapsed plant and powder sugar sporulation on infected pumpkin fruit caused by Phytophthora blight. Photo: Chris Burkhart

Pythium cottony leak is another wet weather disease that has been seen in the field recently. Dense white mycelium will develop at the point of contact with the soil under very wet conditions or when the soil is poorly drained. Aside from the application of mefenoxam at planting, management focuses on improving soil drainage and minimizing contact between the soil and the fruit.


Pythium cottony leak on cucumber. Photo: Steve Bogash

Authors

Integrated vegetable disease management Plant pathogen diagnosis Disease monitoring and forecasting Sustainable crop production

More by Beth K. Gugino, Ph.D.