August is almost here, and with all of the rain and humidity, plant diseases are quickly catching up right as peak summer harvest season begins. This week's update includes an overview of recovering from storm damage, keeping our lungs healthy in smoky air, and crop updates which heavily feature plant disease problems.
Helping plants recover from severe storms
Strong storms across the state earlier this week caused flooding, hail damage, and wind damage on many farms.- If you had areas that flooded, crops that were touched by floodwater shouldn’t be harvested and sold. Flood water is different from pooling rain - use your discretion if the water in your field is just from rain that is pooling on the surface. If flood water moved from elsewhere on your farm into the field, avoid selling produce that touched this water.
- Weather was conducive for many diseases before the severe weather, and looking ahead, while things will cool down, many diseases are still able to infect plants. Damaged plants can be protected using pesticides. Copper is often used to protect from bacteria, who can easily use the moisture and plant wounds to move around. The goal in using any product is to give the plant protection to speed up the healing process.
- Tomatoes, peppers, eggplant: remove fruit that may have been damaged by hail. They will not recover, and are more likely to get postharvest rots if picked and stored. Removing these unmarketable fruit will help that plant put its energy into new fruit.
- Winter squash, pumpkins: Spots from hail can scab over. Sometimes shelf life is impacted, but if damage isn’t severe, they will pull through. Because cucurbits can grow vines back pretty quickly, these crops seem to bounce back in many cases.
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| Hail damage on cabbage. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org |
An effective way to protect your lungs during smoky weather
It seems like every stretch of nice weather this summer has been accompanied by heavy smoke in the air. This week is no exception - we’re looking at multiple days of smoky air, with most of Minnesota in the “unhealthy for everyone” category. Wearing a well fitting respirator with a rating of N95 or better (could be N, P, or R with the number 95, 99, or 100) will help protect your lungs in the field.I recently got an indoor air quality monitor for my house, and realized that on days where the air quality outside is unhealthy, it’s often even worse inside my house. (If you use central AC, the air filter in your HVAC system should help clean the air as long as you change it out on the right schedule, but many older homes, pack sheds, and other farm buildings don't have central air). Just as it’s important to get into the shade or AC to cool down on hot days, it’s important to find some respite in clean air when you’re working outside in smoky conditions, and investing in clean air inside your house, pack shed, or wherever you take breaks is a key way to do so.
Key the Corsi-Rosenthal box. If you go to any retail store, an indoor HEPA-approved air filter is going to cost you a lot of money, and the replaceable filters are often expensive too. If you want to invest in a commercial filter, that’s great! If not, you can make a really effective low cost filter using a box fan and furnace filters. We recently built one at home, and have been very impressed with the results. It took 15-20 minutes to build. When we turn it on, the indoor air quality becomes substantially better in a short period of time - usually in an hour or two. It’s not pretty and it’s sort of bulky, but it works!
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| Indoor air quality graph from Natalie's house before and after turning on the Corsi-Rosenthal box filter |
Crop updates
Basil
Our neighbors in Wisconsin are reporting basil downy mildew is showing up. Many varieties we grow in Minnesota, especially Genovese and sweet varieties, are susceptible and total crop loss is a real possibility. Symptoms first appear on the lowest leaves, the top side will be yellow, and when you flip the leaf over (especially on a dewy morning or humid day), there will be dense, charcoal grey fuzz on the underside of the leaf. Removing these leaves may slow the disease, but the best course of action is to plant a resistant variety next year. These varieties often have DMR in their name, or are coming out of the Rutgers breeding program. If you haven’t tried one of these varieties yet, give one a shot, they grow and taste great. |
| Basil downy mildew. Photo: Rebecca A. Melanson, Mississippi State University Extension, Bugwood.org |
Brassicas
There was a black rot boom this week. We’re now seeing pretty advanced black rot symptoms at most of the farms we’re visiting. If your plants are nearly ready to harvest (or if the harvestable parts are disease-riddled, like in the photo below), prioritize harvesting quickly and chopping / tilling the residue. For growers seeing the first stages of infection, copper products can help to prevent the spread of this disease. Copper should be applied every 1-2 weeks to be effective, especially following major rains. There is some evidence that pairing copper with products that boost the plant’s immune response can improve its efficacy. This publication from Kansas State University covers spray programs for organic Brassicas. |
| Photo: Natalie Hoidal |
Potatoes
A farmer in Southeast Minnesota noticed their potato leaves rotting and dying back, and identified black dot & rhizoctonia after sending a sample to the UMN Plant Disease Clinic. Because they had healthy potatoes in another field, they decided the best course of action was to harvest the diseased field asap as young / fresh potatoes, and chop / till the residue to prevent these pathogens from hopping over to the other field and to nearby solanaceous crops. This brought up an important question about how to cure and store young potatoes that haven’t fully matured with thick skins. UMN Extension post-harvest specialist Cindy Tong shared the following advice:It's been so rainy lately that any potatoes left in the ground after the vines have died or been cut will rot in the field. I suggest lifting the potatoes, letting the skins dry out and cure a little (5 days at a warm temperature with good air circulation), and then refrigerating them. After the skin adheres to the flesh, they can be stored at 40 F but no lower for months unless diseased or injured (rogue injured tubers); they will last for a shorter period of time at 60 F. The main thing you need is air circulation. I know growers who have cured potatoes in the field, just lifting them out of the ground and laying them on top of the soil for a week or two, until the skins heal over. This works as long as it doesn't rain. If you have an empty high tunnel or shed where you can lay out the tubers and have air blow over them, that'll work, too.
Sweet Corn
As more sweet corn crops reach maturity, this is the time of year we hear about animal pests, like birds and raccoons. A big thing to remember whenever you are dealing with animal pests is that they learn. If they learn how delicious your crops are, almost nothing will keep them away. If using some sort of scare to keep animals away from sweet corn, place it before you see damage, and switch out the scare tactic or move it around regularly. This makes it less likely that the animals will learn that the scare you’ve placed is all bark, no bite. Some common scare tactics include: loud noises, balloons, mylar ribbons, and air dancers (as in, the inflatable guys with waving arms at car dealerships).We are also almost to August (?!), a time when we see consistent Corn Earworm pressure. Treating this pest involves having an effective product on green silks, even as those silks grow and get longer over time. The Midwest Vegetable Production Guide highlights more info about management and effective products.
Tomatoes
Many tomatoes in Minnesota have lots of leaf spots, which is par for the course when growing here, especially when it is rainy. As we get later in the season, we sometimes see late blight in Minnesota. This disease doesn’t show up in Minnesota every year, and currently has made it as far as Ontario. We have good weather conditions for it with all the humidity and rain, but if the spores are able to hopscotch their way over to Minnesota is always a big question mark. Signs you might have late blight, as opposed to the much more common and much less destructive early blight are…- Blotchy, green brown spots on the leaves. They have no distinct shape, to me they almost remind me of watercolor paint moving across the leaf.
- These splotches will creep up the leaf and infect the petiole and stem.
- This is one of the rare diseases that can do major damage to green tomatoes. The unripe tomatoes will have brown, amorphous splotches that can appear greasy.
- If it is really humid and wet, the infected areas will produce fuzzy white spores.
- We will keep you updated if this disease is seen any closer to Minnesota, but because it is so severe and can move quickly through plants in the right conditions, keep your eye out when scouting.
Photos of late blight in tomato. Photos: Thirunarayanan Perumal, Banaras Hindu University, Bugwood.org
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