by Katie Navarra
In brassica crops, black rot topped the list as the most problematic disease in 2019, according to Cornell AgriTech vegetable pathologist Chris Smart. At the annual Fruit and Vegetable Growers Conference hosted by Cornell Cooperative Extension Eastern New York Commercial Horticulture, Smart highlighted the most problematic diseases in 2019 and shared predictions for 2020.
“Black rot is systemic, meaning it goes through vines and right into head, so it’s a problem in the field and in storage,” Smart said. “There will be black rot in 2020. I started in 2003 and there’s been black rot in crucifers every single year. Some years, it’s not so bad; others, it’s really bad.”
Caused by bacteria, it is a significant disease in cabbage crops. Symptoms begin with yellowing at the leaf margin followed by V-shaped lesions and then necrosis sets in.
“The years when black rot takes off is when we have warm, wet weather, especially warm wind-blown rains,” she said. “It’s blown from plant to plant and field to field in those water droplets. In years we get hurricane remnants, black rot will spread like wildfire.”
Prevention is the best method of control. Start with clean seed that has been hot water treated and practice good sanitation when growing transplants in a greenhouse. Scouting transplants for early spots is the first line of defense. Smart encouraged farmers who suspect black rot to contact their Extension agents for identification and an action plan. The earlier the bacteria are caught, the better the chances for control.
“A three-year rotation is recommended because the bacteria can survive in those thick brassica stems and last in the soil that long,” she said.
Copper is the go-to method of control as it can slow the spread of bacteria. Once black rot is in a plant, copper won’t help though. About 10 years ago, a Cornell study examined the bacteria’s resistance to copper and none was found. However, Smart said farmers across the state are reporting resistance.
“I have a student interested in exploring copper resistance so if you’re having bacteria issues in the field, report it to local Extension,” she said. “It’s important to know if there’s resistance because you don’t want to hammer on copper if it has no effect.”
An Overview of 2019
Black rot wasn’t the only disease farmers had to contend with last year. Downy mildew was also identified in New York fields. The fungal disease is sometimes confused with black rot, according to Smart. The disease is not a significant pressure every year, but it is problematic in cool, wet seasons of early spring or late autumn and can creep into crops growing in unheated greenhouses.
“Downy mildew lesions have a tan, papery look about them,” she said. “It’s important to know which pathogen you have because control strategies are different.”
Farmers don’t have to contend with fusarium fungus every year, but it was present in 2019. The fungus lives in the soil and is in most fields across the state. When heavy rains splash soil onto forming brassica heads and it’s just the right temperature (80 – 90º F), fusarium appears. It is easily identifiable as an orange coating on cabbage heads.
Crop rotation is often recommended for breaking some disease cycles. However, with white mold this strategy is nearly impossible. The fungus affects 400 different crop species, according to Smart. (Sweet corn is one plant white mold won’t attack.) The fungus spores are visible, looking similar to mouse droppings, Smart explained.
One pathogen becoming increasingly more present is alternaria leaf spot. Smart has seen this fungus appear in crops over the last two years and anticipates it will return in 2020 with greater impacts. Early on the symptomatic dark spots look like many other diseases. The spots turn black in a target-like shape distinguishing this pathogen from others.
“Unless we have a hot, dry summer with no rain, it’ll be back and it’s going to attack all crucifer crops. It’s showing resistance to Quadris and similar chemicals,” she said. “We are working with folks in Georgia and Virginia to determine if what we’re seeing is a new strain or a variation of the fungus species we’ve had before.”
Predictions for 2020
Despite not having a crystal ball to precisely identify diseases to expect in 2020, Smart predicted farmers will contend with several challenging bacterial diseases, especially in tomato and pepper crops. Bacterial diseases are worse in wet seasons as compared to dry seasons, but things like bacterial canker, spot and speck can survive on a farm in the soil.
“We’ve seen several instances where they’re surviving on farm in soil because there is not a long enough rotation,” she said. “It can be seed-borne, so it can arrive on seed. Transplants packed together in a greenhouse watered with overhead irrigation is the perfect situation for spreading the bacteria.”
Bacterial speck on tomatoes wasn’t problematic until an outbreak occurred in 2009. Smart has seen it every year since. Small, speck-sized raised lesions form on the tomato fruit, making it unmarketable. A yellow halo surrounding brown spots on the leaf distinguish a bacterial disease from a fungus.
“Plants can get necrotic leaf margins from a lot of different things, including drought stress. Knowing if it’s an abiotic or biotic issue is really critical,” Smart said. “Like black rot on cabbage, canker can survive inside the phylum of the plant and under the right conditions can move quickly through the plant.” It prefers cooler weather.
Smart named bacterial spot as the pathogen to watch in 2020. It emerged about five years ago and is increasing in prevalence across the state. It attacks both tomatoes and peppers and can move between crops. Sanitation is key.
“Disinfecting greenhouses and trellising stakes is critical because the cold will not kill the bacteria, as they survive down to -80º,” she said. “The bacteria survive on metal, plastic, wood or any material. Spray the organic matter off stakes first, then put in a livestock trough with sanitizer.”
As problematic as bacterial canker, spot and speck are, phytophthora blight is even more challenging. The disease can last in the soil at least 10 years and is expected to be more prevalent in cucurbits going forward.
“Host resistance is one minor component of phytophthora blight management. For example, the Paladin pepper has resistance to it,” Smart said. “Keeping pathogens out of disease-free fields and promoting good drainage as much as possible is key. Also, rotating products to avoid reliance on chemicals with resistance issues.”
Phytophthora blight thrives in standing water, so promote subsoil drainage through tillage or other practices that prevent water pooling. Growing on plastic helps prevent it in upright crops, but in vine crops where the fruit is on plastic, water collecting on the plastic will have the same problem as pooling on ground.
“One of the worst things I have to do is tell people that they have it,” she said. You have to do your best to avoid it.”
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