Written By: Emily Cabrera, IPM Communications Coordinator
Expert Source: Bhabesh Dutta, Extension Plant Pathologist
A particularly nasty fungus is making a comeback across vegetable fields in the South but research efforts by UGA specialists hope to give farmers the refined tools they need to stop it in its tracks.
Alternaria Leaf Spot (ALS), also referred to as Alternaria blight and head rot (ABHR), is a fungal disease that affects brassicas and is especially hard-hitting in kale, broccoli, and cabbage. The presence of this fungal disease has been around a long time but has become more critical in the last few years because it has become completely resistant to some of the most effective fungicides that were used to manage the disease.
University of Georgia Extension Plant Pathologist, Bhabesh Dutta, has been working the last two years on research looking at this emerging issue to determine whether or not we are dealing with a novel species or one that has simply mutated over time through repeated use of a specific fungicide.
“We found that ABHR causing fungi are distinct from conventionally known causal agents of the same disease in brassicas and more-so these fungi are highly-to-moderately resistant to commonly used fungicides,” said Dutta.
The old fungicides he is referring to are in a chemical family called Strobilurins, which are in the Fungicide Resistance Action Committee (FRAC) Group 11. Strobilurins are extremely useful in controlling a broad spectrum of common vegetable pathogens, and due to our warmer, humid climate, growers have inadvertently overused this chemical in an effort to prevent a wide range of fungal diseases in brassicas.
Due to this finding, the Georgia Agriculture Commission previously awarded Dutta two years of funding (2018 and 2019) to find immediate management options for GA brassica growers. Dutta has pulled together a multi-state team focusing on finding population structure, pathogen distribution, fungicide resistance and effective cultural and chemical methods for controlling this disease.
“This disease has been posing a threat to the brassica industry along the east coast but somehow is not so prevalent in other parts of the US” said Dutta. The 2018 Farm Gate value for important brassica crops in Georgia: cabbage ($25 million), broccoli ($20 million), and kale ($12 million).
“The intriguing nature of the pathogen and limited management options motivate researchers to come up with innovative and sustainable methods to manage ABHR,” he said.
“Alternaria is a foliar pathogen. Symptoms first appear on older leaves as small, dark spots that gradually enlarge with concentric rings. As the disease progresses, younger leaves can also become infected. In severe cases, infection can occur that results in rot on heads” explained Dutta.
“Currently, we recommend rotating fungicides from FRAC Group 7 (carboxamides), FRAC Group 3 (Demethylation-inhibiting) and FRAC Group 9 (anilinopyrimidines) for resistance management,” said Dutta, “we also recommend growers to follow good cultural practices year after year”.
Cultural practices should definitely incorporate a strong weed management program, as the pathogen survives in weeds too” said Dutta.
“Most of the brassica crops in GA are over-head irrigated that promotes pathogen dispersal and also increases the period of leaf-wetness. Leaf wetness period drives the pathogen infection and colonization” said Dutta. So, when possible, growers are encouraged to utilize drip irrigation to minimize the spread.
“It is also important for growers not to over-fertilize, as it makes the crop more susceptible to ABHR” explained Dutta.
“We strongly encourage growers to select tolerant varieties” urged Dutta. ‘Emerald Crown’ broccoli has shown some tolerance to the pathogen and is considered the ‘gold standard’ for commercial growers in Georgia.
“We will continue to make information available as we learn more about this new ABHR causing fungus,” said Dutta, “but for now, just be diligent to minimize the onset by following the practices listed here.”
If you are interested in learning more about irrigation management for brassicas, check out Extension circular #1169. You can learn more about fruit and vegetable production in Georgia by visiting: https://extension.uga.edu/topic-areas/lawn-garden-landscapes/fruits-vegetables.html.