By Erin Schneider, Hilltop Community Farm
with Drs. Amanda Gevens and Ruth Genger, UW Plant Pathology Department
“What’s the weather like?” I asked Rob, my husband, farm partner, and dedicated weather enthusiast.
“Well, the cooler, damper conditions that prevailed in June and that we’ve seen periodically thus far in July, tells me that it’s cloudy with a chance of spores.”
I did a double take, then realized, “Oh, you must have tuned into Blitecast and caught the latest late blight update from Dr. Gevens, Extension Plant Pathologist in Potatoes and Vegetables with the UW Vegetable Pathology team.”
“Of course, what did you think, I was talking about,” Rob responded, “the latest Madden-Julian Oscillation?”
Late blight, that ferocious sporulating nightmare of a plant disease that caused the 1848 Irish potato famine and fueled massive emigration from Ireland, still sends shivers down the spines of any vegetable grower. Late blight has been found in Adams, Waushara, Wood and Marquette Counties in Wisconsin this year, so it’s time to be on the lookout.
The disease is caused by the fungus-like microorganism Phytophthora infestans, – literally “plant infesting destroyer” – a member of a large group of plant pathogens that can infect a wide range of trees, vegetables and fruits. P. infestans, as it’s known to plant pathologists, can infect potatoes and tomatoes, as well as some nightshade weeds and petunia.
Late blight can come from infected seed potatoes or tomato transplants, potato cull piles and compost piles, volunteer plants, and is also spread aerially. The disease can devastate fields of potatoes and tomatoes we worked so hard to bring to fruition. None of us want to be in the position to tell our CSA members, customers, and grocers – sorry – no potatoes and tomatoes this year.
Fortunately, like the National Weather Service’s Storm Prediction Center, there are tools that farmers can utilize to forecast when an ‘inoculum storm’ would most likely infect given knowledge of the pathogen’s disease cycle. I spoke with Dr. Amanda Gevens of the UW-Madison Plant Pathology Department to learn more about disease forecasting and management options for late blight.
For on-farm management, Dr. Gevens recommends starting with good sanitation. Don’t keep discarded potatoes in cull piles, or compost infected tomato plants. Instead of saving seed potatoes, source certified seed potatoes, and consider planting shorter season varieties to avoid the pathogen. Raising one’s own tomato transplants can greatly aid in limiting disease introduction from other regions. High tunnels or greenhouses can limit initial infection by aerial ‘spore shower’ and has been shown to reduce late blight infection or greatly slow its onset.
Resistant varieties are always a boon to disease management. A handful of tomato varieties have significant resistance (but not immunity) to current late blight strains. These include; Iron Lady, Defiant, Mt. Magic, Plum Regal, and Matt’s Wild Cherry. In potatoes, few disease resistant options are available, but varieties with some tolerance include Satina, Jacqueline Lee, Sebago, Serran, and Kennebec.
Knowing the risk of infection helps us stay ahead of the pathogen, which is where disease forecasting models like Blitecast come in. Dr Gevens shared that, “‘blightcasting’ involves looking at temperature and periods of high relative humidity and assigning disease severity or risk values to each day. Research has shown that after a certain accumulation of ‘risk’ or severity values, disease is likely to occur if the pathogen is present and you have susceptible crops. While we can’t yet forecast where spores would be, temperature and relative humidity are helpful in managing pathogen persistence.”
Scouting for disease symptoms is another critical component of management. As a progressive farmer once said, “the best thing I can see in my field is my shadow”. Know the leaf disease symptoms on tomato and potato, and, in Wisconsin, send in samples for free testing (see below for details).
While not fun for any grower, fungicides can be part of the management solution. For organic growers, copper sprays are the most tried and true in managing this disease. Not all copper fungicides are approved for use in organic management, so check with your certifier or the OMRI list (http://www.omri.org/). Copper acts as a contact fungicide, so must be applied regularly to cover new foliage – it will not control an existing infection. Be mindful that copper accumulates in the soil and will also kill beneficial fungi. To spray or not to spray, as with many management decisions, involves trade offs. There is one other fungicide that has shown good results in limiting late blight in organic systems – EF400. This product isn’t OMRI approved but has been acceptable to certifying agencies.
Even less fun is the prospect of having to destroy your crop. “If you can’t shut down sporulation,” Dr. Gevens states, “it is best stewardship to shut down the crop. Once the plants are dead, the pathogen is dead and you can limit the production of more inoculum/spores for the remainder of your own farm and for your neighboring farms. Late blight is a community disease and it takes all growers of susceptible plants to participate in its management.”
Once confirmed in an area, the grower grapevine kicks in. Communication and planning beyond the farm gate help contribute to farmers’ ability to stave off damage. Wisconsin growers can report late blight to Dr. Gevens’ team and be assured of anonymity. Dr. Gevens relays, “When late blight has been confirmed in your ‘area’ of production the risk for infection increases on all farms. A sporulating lesion or lesions indicates that the pathogen was present and active for likely about one week. And, in that time, spores can be formed and could have moved aerially or through splash dispersal to new areas within a field and to new fields in an estimated 40 mile radius. Other fields may have already been infected but not yet observed for diseases. I’m conservative in my management recommendations for late blight in WI because the disease can be so destructive to potato and tomato crops impacting the economy and reputation of the growers of vegetables in our state.”
“Few diseases have the aggressiveness and longevity of late blight,” Dr. Gevens states. “I am fascinated by its ‘strength’, its impact on agriculture and history, and by the pathogen’s plasticity. It continues to outwit some brilliant growers and researchers since 1850.”
Dr. Gevens welcomes farmers of all scales, sizes, and production types, to be included in the UWEX Vegetable Crop Updates newsletter, which includes the disease forecasting information and reports for WI and the continental U.S. Growers are encouraged to send in pictures and samples to your local Extension agents if you need help. Knowing where the disease is and when it needs to be more actively managed can save on sprays and grower anxiety.
This article was posted in Blog Posts and tagged Late blight, organic potato, tuber diseases.