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Choosing resistant varieties in 2020 for common 2019 diseases

Author: Natalie Hoidal, University of Minnesota Extension

I've had multiple conversations this week with farmers who have told me that after feeling burnt out from last year's growing season, they are starting to feel excited to open their seed catalogs again. This is an exciting time for planning - from routine logistics to big ideas. Maybe you've already ordered your seeds, but if not, consider ordering varieties that are resistant to some of the major diseases we saw in Minnesota over the 2019 growing season.

Before jumping into varieties and diseases, I want to define a couple of concepts and highlight some resources.

Resistant: A plant with resistance has certain characteristics that make pathogens less likely to enter the plant or to reproduce on or in the plant. Sometimes varieties will be labeled as partially or moderately resistant. 

Tolerant: A plant with tolerance can still become infected with a pathogen, but the damage will be less severe than a susceptible plant. 

Even when you've selected varieties with tolerance or resistance to specific pathogens, it's still very important to practice excellent sanitation and other cultural control methods.

Two places I like to look for resistance information are the Cornell Resistant Varieties page and Johnny's online catalog, which has a nice filtering system for many diseases. (This is not an endorsement for buying seeds from one company vs. another - these are just good places to start for figuring out which varieties might have resistance or tolerance to specific diseases). There can be errors in these pages, so it's always a good idea to double check by calling the breeder or the seed company.

Here are a few of the most common diseases I saw in 2019. This is by no means an exhaustive list - these are just my observations from last year's growing season. The best approach is to check your own records for any diseases you saw last year on your farm, as those may be the most likely diseases to show up again this year. If you have neighbors who grow vegetables, it's not a bad idea to consult with them as well.


Alternaria leaf spot. Photo: Natalie Hoidal
Black rot (Xanthamonas campestris pv campestris) was a major disease of brassicas in 2019. While there are a few tolerant cabbage varieties (see this excellent presentation for an overview), we do not know of any resistance of tolerance in broccoli, brussels sprouts, cauliflower, or kale. You've all done an excellent job of voicing the need for more research on this front, and I'm optimistic about some potential collaborations that are coming together.

Alternaria leaf spot was widespread in 2019, and relatively new to many growers. While some of you had experienced Alternaria before 2019, it seemed to be much more widespread this year. Unfortunately, there are no varieties on the market with robust resistance to this disease (it's actually caused by three separate, but closely related pathogens).


Bacterial Speck, NH
Tomatoes are notorious for diseases. It's pretty common to experience early blight, Septoria, and Fusarium, and there are indeed varieties with varying levels of resistance to these diseases. These are foliar diseases that can reduce yields, but often do not impact fruit quality if you're practicing good sanitation and preventative management.

Leaf Mold, NH
Two diseases I saw quite a bit of last year with major impacts to fruit quality were bacterial leaf spot (Xanthomonas spp.) and bacterial leaf speck (Pseudomonas syringae pv. tomato). These diseases make the fruit of tomatoes unmarketable, and thus they have significant economic impacts to growers. There are not many resistant varieties available, but there are a few.
High tunnel tomatoes tend to have different diseases than field-grown tomatoes. In particular, leaf mold seems to be a problem for almost all high tunnel growers. This fact sheet from Cornell is a great resource for leaf mold management, and towards the end includes a list of varieties with leaf mold resistance.


Anthracnose, NH

I saw Anthracnose (Colletotrichum orbiculare) on many farms this summer. Honeydew tends to be very susceptible to Anthracnose, as does watermelon and some cucumber varieties. Cantaloupe is generally less susceptible, but occasionally farmers will see fruit damage. When selecting watermelon varieties, it's easy to be misled. There are 2 races of Anthracnose, and watermelon is most often affected by race 2. Many watermelon cultivars are marketed as being resistant to race 1, but this is unimportant if race 2 is present on your farm. Unfortunately there are no commercially available race 2 resistant watermelons. There are quite a few resistant cucumbers available.

Peppers & Pumpkins

Phytophthora, NH
Phytophthora (Phytophthora capsici) is an emerging disease in the Midwest that affects cucurbits and solanaceous crops. Since it affects two families, crop rotation is especially important. There are no available resistant cucurbits, but it has been reported that at least for small pumpkins and squash, varieties with harder rinds may be less susceptible. There are some resistant bell peppers. The resistant peppers do tend to experience some silvering, but you'll likely agree that this is better than rotting fruit. 

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