Jul 9, 2015
Late blight, downy mildew confirmed on East Coast

On Monday, late blight was confirmed in two tomato fields in Morris County in Northern New Jersey. These are the first reports on tomato in the mid-Atlantic and Northeast regions.

Previous reports, the closest of which were in western NY, were on potato. One of these samples has already been characterized as US-23, the predominant genotype since 2012. Since the major outbreak in 2009, in PA we seen late blight very year and have confirmed it as early as 17 May in 2010 and as late in the season as 12 July 2012.

Symptoms on tomato and potato are very similar and any plant growth stage is susceptible. It is important to be scouting your crop regularly looking for irregularly shaped water-soaked lesions that are initially pale green before turning gray- brown. Under humid conditions, the lesions on the underside of the leaf will sporulate giving them a white fuzzy appearance. The lesions will tend to develop on the upper to middle part of the plant as opposed to early blight and Septoria leaf spot (tomato only) that start on the lower leaves and progress up the plant.

Keep in mind, unless you have planted a late blight resistant tomato cultivar (e.g. Plum Regal, Mountain Magic, Mountain Merit, Defiant, Iron Lady) the only way to manage late blight during the growing season when conditions are favorable for disease is with the use of protectant and/or late blight specific fungicides. Without careful scouting and the use of fungicides, you could potentially lose your crop in as few as 5 days. Given the persistent high relative humidity and frequent rain events, protectant fungicides are recommended. For organic production, copper is still one of the most effective in-season tools and is most effective when applied before symptoms are observed. Copper tank mixed with Actinovate and alternated with copper tank mixed with Regalia would be one suggested organic program. Since Regalia functions to boost the plants defense system, initiating applications early in the season is recommended.

If you suspect late blight on your farm, please contact your local Penn State Extension Office or let me know via email 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. Additional images of late blight on tomatoes and potatoes can be found at the Penn State Extension Vegetable and Small Fruit website under the Vegetable Disease Images link on the homepage. 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 http://usablight.org.

Yesterday downy mildew was confirmed on cucumber in Berks County. This is the first confirmed report in Pennsylvania, however there are likely more infected fields around so scout yours now if you haven’t already.

Infected cucumber fields are the primary sources of inoculum both north and northwest of PA in MI, OH, NY and Ontario while downy mildew has been confirmed on a more diverse range of cucurbit hosts to the south. These include watermelon in DE, MD, SC and GA as well as acorn, summer, and butternut squash and cantaloupe in SC. The vast majority of reports in the south however have been on cucumber.

Recent conditions have been very favorable for downy mildew infection in PA and the continued chance of rain and storms in the long range forecast will likely continue to put PA fields at risk.

If you are not already doing so, it is important to be regularly scouting your cucurbit fields especially cucumbers, cantaloupe and watermelon regularly for downy mildew. It takes several days for symptoms to develop after infection and conditions this past weekend were very favorable for disease development. Look at both the upper and lower leaf surface. Purplish-gray sporulation on the lower leaf surface is diagnostic for downy mildew. If you have been applying protectant fungicides then the sporulation may not be very obvious.

At this time it is recommended that you, at the very least, use a protectant fungicide program on your cucumbers and cantaloupe if you are not already using one and consider expanding it to include all your cucurbit crops due to the reports on watermelon in DE and MD. Also consider including a copper-based product if you are also concerned about angular leaf spot. For organic production, copper-based products are most effective and when alternated with Serenade have reduced disease severity. Oxidate is effective at killing the spores that it comes in contact with when applied but has not residual activity. Other possible rotational partners with copper are Regalia and Actinovate. Under favorable conditions, it can be very difficult to manage downy mildew either conventionally or organically. If you have succession planting, disking under or destroying the crop residue as soon as you are done with harvest will reduce disease pressure on the other plantings.

For the latest information on outbreaks and to receive email or text alerts please visit theCucurbit 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 to let me know either by email or by phone at 814-865-7328 or contact your local Penn State Cooperative Extension Office.

— By Beth K. Gugino, Associate Professor Vegetable Pathology