Managing fall Brassica diseases: Alternaria leaf spot and Black rot

Author: Natalie Hoidal, Extension Educator, Local Foods and Vegetable Production

At this point in the season, at least for diversified vegetable farms, the busiest part of the season is behind us. There’s still a lot of work to be done, but I encourage you all to take some time to make a plan for fall Brassica management, and in particular disease prevention. This season has been generally dry with fewer disease problems than normal, but we’re starting to see pressure from black rot (Xanthomonas campestris pv. campestris) and Alternaria leaf spot (Alternaria spp.)

What can you do at this point in the season to prevent problems?

Late summer Brassicas are often a source of inoculum for fall Brassicas. Photo: NH

What can you spray?

I reviewed twelve recent trials assessing the impact of various fungicides on black rot and Alternaria leaf spot in Brassicas, and compiled the results of each product into a table for comparison (see below). The data is complicated. A few of the conventional products provided significant control and yield protection, but many did not. The most promising and widely recommended products available to non-certified organic growers include are listed in the Midwest Vegetable Production Guide for Commercial Growers.

None of the OMRI approved products provided statistically significant control, and the variation between studies is sufficient that it’s difficult to make consistent recommendations to growers. For example, in Jordan & Gevens (2019), yield increased ~1.2-1.5 fold with treatment combinations of Stargus, Regalia, Badge, and Nufilm. However, in Miller (2019), the same products caused a yield reduction compared to untreated controls.

Often, fungicide products do significantly reduce disease incidence and severity, but this rarely translates into improved yields. Why? One of the common hypotheses for why plant defense stimulating products reduce pathogen load but do not help yields is that it’s energy intensive for plants to produce defense compounds. While they are busy fighting pathogens, they have less energy to invest in yields.

So what’s the takeaway? There is enough variability in the data that it’s hard to make a sound product recommendation.While I do see compelling enough evidence to keep trialing some of these products, there is not enough evidence to suggest that any of them would help to improve yields all or even most of the time.

What are the alternatives?

With very few fungicide options, especially for organic growers, we need to put our eggs into other baskets. The absolute most important thing that farmers can do is practice good sanitation to prevent moving pathogens from field to field, and especially from older diseased plantings into younger, healthier plantings.

1. How are you moving through your fields?

No product provides a miracle cure that will provide excellent control without a cost. Most of the available fungicide products, especially those that are OMRI-approved, work best when these diseases are relatively under control already. This means that cultural control and prevention are still key pieces of management.

It’s really easy to say what needs to be done; it’s a lot harder to implement these practices. Use a four year crop rotation, don’t move pathogens from field to field, practice excellent weed control and sanitation, build organic matter and promote good drainage in your soils, and prevent splashing from the soil. Simple right? Not at all. It can be overwhelming to think about how to implement all of these strategies at once, but even just using a few can make a big difference.

In particular, at this point in the season you likely have a summer succession of broccoli (or any Brassica) that’s either about ready to harvest or already harvested that has some disease pressure, as well as some plots with young healthy seedlings. I’ve watched multiple farmers walk from fields with high disease pressure into clean fields; this is a great way to spread pathogens from field to field. By starting your day in your earliest and healthiest successions with clean clothes, tools, and shoes, and ending your day in your most diseased fields, you can have a substantial impact on reducing pathogen spread.

2. Mulch

I have cited this study before, but I’ll cite it again. Scheuflele et al. (2013) compared various bio fungicide treatments alongside mulching options for organic management of Alternaria. While none of the fungicide treatments were effectie, mulch had a statistically signifcnat effect on disease incidence. Plastic mulch and BioTelo reduced disease pressure by about half (though it was not statistically significant due to variation), and straw mulch reduced it significantly by more than six fold.

3. Variety selection

There is still a lot of work to be done with variety selection, and it is too late this year to change varieties. However, there is one exciting finding from these fungicide trials that I wanted to share. de Silva & Dutta (2019) studied Alternaria incidence in two cabbage varieties: Eastern Crown and Imperial. They found that Imperial had double the disease incidence and significantly lower yields than Eastern Crown.

In 2020, we’ll be trialing about 40 broccoli varieties for Alternaria and black rot tolerance, and we’ll be looking for grower collaborators to grow 3-4 varieties each.

Table of products included in 2018 & 2019 trials & brief overview of results

**Table is not perfect due to the volume of information. All combinations of products and rates are included in the lefthand column, but frequency of applications is not reported. In most trials products were applied between 5-10 times throughout the season. All original studies are cited below.

***Not all of these products are labeled for use on vegetables in Minnesota (i.e. legal to use). If you are interested in trying any of these products make sure to check the label, and if you're organic work with your organic certifier.

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Sources:

De Silva & Dutta. 2020. Plant Disease Management Reports 14:V112, https://www-plantmanagementnetwork-org.ezp2.lib.umn.edu/pub/trial/PDMR/reports/2020/V112.pdf

Hoepting, 2019. Plant Disease Management Reports 13:V170 , https://www-plantmanagementnetwork-org.ezp2.lib.umn.edu/pub/trial/PDMR/reports/2019/V170.pdf

Jordan & Gevens, 2020. Plant Disease Management Reports 14: V198, http://www.plantmanagementnetwork.org.ezp2.lib.umn.edu/pub/trial/PDMR/volume14/abstracts/V198.asp

Scheuflele et al. 2013. http://www.hort.cornell.edu/expo/proceedings/2013/Cole%20Crops/Cole%20Crops%20Scheufele%20Alternaria.pdf

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