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Sightings from the field: week of August 26-30, 2019

Author: Natalie Hoidal, UMN Extension horticulture educator

Alternaria (black / brown leaf spot) in Cole crops - Rice and Chisago counties

We've seen Alternaria leaf spot showing up on cole crops on a few different farms this week. Alternaria leaf spot is caused by three different strains of the Alternaria fungus: Alternaria brassicicola, A. brassicae, and A. raphani. It can be identified by the target-like appearance of spots on the leaves, forming concentric rings. Leaf spots tend to appear first on the lower leaves, and will often break open and fall out. If you're seeing Alternaria for the first time in your fields, the disease likely came on infected seed. 

Alternaria leaf spot on a broccoli leaf. Photo: Andrew Hanson-Pierre

For conventional growers, the Midwest Vegetable Production Guide contains a list of effective fungicides. For organic growers, options are limited, with organic fungicide products showing limited success. Practicing good sanitation is key. For next year's planning, make sure to rotate your cole crops and call your seed company to make sure they can confirm that their seed is disease-free. If this is your first year with Alternaria, consider calling your seed company to let them know. Mulch has been shown to be effective at reducing Alternaria incidence, as has biochar - for more information on this research, see "Can Alternaria Leaf Spot be Managed Organically?" by Scheufele et al., 2013. 

Agritourism with sunflowers, Chisago County

These farmers were transitioning out of intensive vegetable production and looking for ways to diversify their farm income. They had some sunflowers out by the road a couple of years ago and noticed that people were stopping to take photos. This year they succession planted two fields of sunflowers away from the road and advertised to local photographers, who pay a small fee to bring clients to the farm for photo shoots (just in time for senior photos). By using succession planting, they're able to extend the flowering period for a few weeks, and the seeds are then used for their animals. North Dakota has taken this idea to a whole new level and turned it into an official state tourism campaign. 

Photo: Natalie Hoidal

Blossom end rot
We're continuing to see blossom end rot around the state. Consistent moisture is key to preventing blossom end rot - even if plants don't seem water stressed, consider minimal irrigation each day. Investing in a moisture sensor is a good way to make sure that you're maintaining consistent water levels and avoid over-watering. 

Photo: Natalie Hoidal

What are you seeing in your fields this week? You are always welcome to reach out to Natalie (vegetables - and Annie (fruit - for assistance with diagnosis or advice. If you're seeing disease symtpoms, consider sending a sample to the Plant Disease Clinic.

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