Warm, wet weather favors early appearance of several vegetable diseases

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Based on the disease outbreaks that we are seeing, I could swear that it is the middle of August. The wet weather early on in the growing season coupled with some warm nights has set us up for trouble on several fronts. I fully anticipate that this season will be tough on growers who are trying to rein in spots, blights, mildews and rots. Here’s the scorecard:


Cercospora is a fungal pathogen that has been found in two areas of the state. This pathogen causes a blight that affects both old and young foliage. Small, circular lesions occur at first, but expand to affect large portions of the leaves. Cercospora can develop quickly and is often more difficult to control than alternaria blight. When cercospora develops on the petioles of the carrots they become weakened and yield is negatively impacted. Fields with this disease can be treated with fungicides including chlorothalonil (ex. Bravo) and the strobilurins (Quadris, Cabrio, Pristine, or Flint). Remember that the strobilurin products are similar in their activity and should not be overused. A good fungicide program will alternate applications of chlorothalonil with a strobilurin. The TOM-CAST disease forecasting system can be used to time sprays for both Alternaria and Cercospora.

Bacterial blight has also been confirmed in a processing carrot field. This particular sample was especially disturbing because of the extensive amount of infection and blighting of the young, expanding leaf tissue. In some instances, bacterial blight on carrot can look dark and almost oily. However, this disease can also appear as a light brown blight that can resemble Alternaria. Bacterial blight can infect young foliage much like Cercospora blight whereas Alternaria blight infects older, senescing leaf tissue. Bacterial blight cannot be managed with those products used to limit Alternaria and Cercospora. Only copper-based products limit bacterial blight. Copper sprays must be applied frequently (minimum every seven days) with thorough coverage of the foliage (no skimping on the amount of water in the spray tank). The TOM-CAST program cannot be used to time sprays for bacterial blight. Copper sprays are limited in what they can offer for bacterial blight of carrot. If the spray interval is stretched beyond the seven days, then the copper applications cannot offer help. The coppers can help limit the spread of the disease and protect the newly emerging foliage. Coppers can reduce the severity of the disease, but they will not cure the blight.


Rust and purple spot were problems early on this spring, so I don’t expect them to go away this summer. The TOM-CAST system can be used to time fungicide sprays for purple spot on asparagus fern. The primary fungicide that is recommended for use against purple spot is Bravo. Rust is controlled through applications of Folicur that can be alternated with Bravo (chlorothalonil). The TOM-CAST program cannot be used to time fungicide sprays for rust. In cases where disease is not yet established, Mancozeb can be used in place of Bravo to maintain a level of protection. However, if disease is detected, Bravo will be better able to limit both purple spot and rust and will weather better than Mancozeb during wet periods. Asparagus growers should do everything they can do to protect their fern and maintain its vigor into the fall. Green, healthy fern will produce the carbohydrates needed for yields next year.

Pumpkins and hard squash

Powdery mildew has been found in two locations within the state. Yes, this is a full month earlier than what we would expect. However, it is not the first time that we’ve had powdery mildew occur prior to the Fourth of July. Scouting will be the key in being able to delay powdery mildew fungicide applications. By scouting, I mean walking the field and looking for the white powdery colonies on the lower leaves and the undersides of the leaves. Scouting via a pick-up truck going down the road at 50 mph doesn’t count.

Once the very first powdery mildew colonies are observed, a spray program should begin immediately. Growers differ in the types of products that provide a good level of control. While some growers are satisfied with Nova and the strobilurins (Cabrio, Flint, Quadris, and Pristine), others note that a combination of Bravo + Topsin works well. Part of the reason why the reports vary so much is that some of our powdery mildew isolates may have developed resistance to one or more of our commonly used fungicides. The key to a successful powdery mildew program is to monitor the level of disease in your field carefully to determine whether your fungicide program is working and if it isn’t, change the products you’re using.


Phytophthora crown rot is affecting some pepper fields in the state. We
have two field plots testing a commercially resistant cultivar against a
susceptible cultivar. Both the resistant and susceptible cultivars are
being sprayed with several different fungicide programs. This research
is being conducted in my program by Jennifer Foster, a Master’s student
from Ontario. At this point in time, all fungicide programs are
struggling to protect the susceptible ‘Red Knight’ peppers even though
we are using raised plant beds. We chose ‘Paladin’ as our resistant
variety to screen in the field since it held up better than ‘Aristotle’
in our greenhouse experiments. At this juncture, the ‘Paladin’ peppers
are surviving irrespective of the fungicide program used. I expect that
may change as the trial continues. I’ll provide updates as the season
progresses. Registered fungicides for use against Phytophthora in
peppers include Tanos, Ridomil Gold Copper, Phostrol (or Fosphite,
Prophyt), and Acrobat (Forum). Remember, all sprays should be targeted
to the base of the plant to protect the lower stem and crown portion.

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