Vegetable Disease Update – June 15, 2018

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Tomato Late Blight
Tomato transplants were found at a garden center in northeast Pennsylvania last Friday (June 8) that were infected with late blight. We don’t know the origin of these transplants, and I understand that none were sold. However, the occurrence warrants extra vigilance of any tomato plantings to scout for late blight. Jerry Brust wrote a good article on recognizing the symptoms, which was published in last week’s WCU.

Cucurbit Powdery Mildew
Please read labels carefully as some of the fungicides mentioned in this article are not labelled on all cucurbits.

In planning your spray programs for powdery mildew on cucurbits, remember that many products that are labeled for this disease, are not effective because of the existence of fungicide resistance. The powdery mildew pathogen Podosphaera xanthii, is highly prone to the development of resistance and has lost sensitivity (become resistant) to some recently registered fungicides. To manage powdery mildew, begin with good cultural practices – especially the use of resistant cultivars. In a spray program, alternate targeted fungicides in different FRAC groups, apply fungicides at manufacturer’s recommended rate (don’t cut the rate), tank-mix with a fungicide with different mode-of-action (or use combination product), and don’t apply at-risk fungicides if powdery mildew sporulation covers more than 20% of the leaf surfaces.

Currently the following targeted fungicides are NOT effective for managing powdery mildew (resistance in pathogen population is high): Topsin M (FRAC 1, Benzimidazole), and Cabrio, Quadris, and others (FRAC 11, QoI fungicides).

Resistance exists, but the following fungicides may be used judiciously, to a limited extent, in a rotational program: Rally, Procure, Folicur, (FRAC 3, DMI fungicides); Pristine (FRAC 7 + 11), Fontelis, Xemium, Aprovia (FRAC 7, SDHI fungicides). Luna is also a FRAC 7 fungicide, however it isn’t cross resistant with the other FRAC 7 group.

Fungicides that still have good efficacy in our area in the past two years are: Quintec (FRAC 13) and Vivando (FRAC U8). Luna Experience and Quintec alternated with Procure are alternations that I have tested and performed well. Unfortunately, resistance to Torino (FRAC U6) has now been documented in the eastern U.S. and it hasn’t performed well in my trials.