Defend Your Cole Crops from Three Critical Diseases

Black rot, Alternaria leaf spot, and downy mildew are all known to adversely affect cole crops, such as broccoli, cabbage, and cauliflower throughout the U.S.

Downy mildew is caused by the oomycete water mold Hyaloperonospora parasitica, while Alternaria leaf spot is the result of the fungus Alternaria brassicicola, and black rot is caused by the bacterium Xanthomonas campestris pv. campestris.

Chris Smart, a Professor in the Department of Plant Pathology at Cornell University, provides information on symptoms of all three diseases as well as control measures to prevent them from damaging your cole crops.

Downy Mildew

Leaf symptoms of downy mildew in cole crops start as yellow areas on the upper surface of the leaf with white pathogen growth on the underside. Photo courtesy of Smart Lab, Cornell University

In the Northeast, downy mildew is most common on cole crop seedlings during the early spring in greenhouses and in open field, as well as later in the season when the weather is cool and wet with extended dew periods.

The pathogen survives in the soil and on crop debris, and requires water and leaf wetness to infect. The spores blow from plant to plant and from adjacent fields and also can produce an overwintering spore that survives in the soil. Smart says the disease has been observed quite commonly in New York for the past four years.

Symptoms on the seedlings include discolored spots on the cotyledons, which may eventually turn yellow and die. Leaf symptoms start as yellow areas on the upper surface of the leaf with white pathogen growth on the underside. Infected areas enlarge and turn papery and tan in color as symptoms progress.

Irregular spotting can be seen on cabbage, cauliflower, and broccoli heads, which make the crop unmarketable.

Some varieties of cabbage are more susceptible to downy mildew than others, so Smart says selecting tolerant varieties is a potential cultural control.

“Also, in the spring, downy mildew is occasionally seen during transplant production, and anything that can be done to reduce leaf wetness during transplant production helps reduce its incidence,” she explains.

Regarding chemical controls, Smart has done studies in her lab for cabbage and found that many labeled products, including those with an active ingredient of chlorothalonil, copper hydroxide, pyraclostrobin, or phosphorus acid, were effective.

Alternaria Leaf Spot

Alternaria leaf spot symptoms begin as dark spots on leaves, which get larger over time, causing target-shaped lesions on the leaves where dark spores are produced. Photo courtesy of Smart Lab, Cornell University

Alternaria leaf spot is common throughout the production season, but is most commonly seen later in the season, according to Smart. The fungal pathogen likes cool, wet conditions and can be transmitted through infected or infested seed, infested debris in soil, infected weeds, and nearby infected cruciferous crops.

Alternaria leaf spot symptoms begin as dark spots on leaves, which get larger over time, causing target-shaped lesions on the leaves where dark spores are produced. When plants are severely infected, defoliation of the outer plant leaves may occur, leading to overall quality reduction and significant yield loss.

Using clean seed and practicing crop rotation are two cultural controls, and Smart says the pathogen can be controlled using fungicides with the active ingredients chlorothalonil and azoxystrobin.

“We have recently done fungicide resistance studies on about 50 diverse isolates collected from diseased plants across the state of New York, and all isolates were found to be sensitive to both the active ingredients listed above,” she says.

Black Rot

Black rot can become established through planting infected seedlings or transplants, or planting in fields where the disease was a problem in previous seasons. Photo courtesy of Smart Lab, Cornell University

Black rot is a significant problem for cabbage in New York and can be seen at any time during the season. While the pathogen is present every year, Smart says large outbreaks do not happen annually, but rather when warm, wet rains occur that spread the bacteria rapidly in the field.

The pathogen can overwinter in and on the seed and in plant debris left in the field. It can become established through planting infected seedlings or transplants, or planting in fields where the disease was a problem in previous seasons.

Cole crops can be affected by black rot at any stage of growth, and the bacterium usually enters through water pores in the plant around leaf margins.

After the bacteria enters, the leaf tissue turns yellow and infection can progress toward the back of the leaf and form a V-shaped pattern in the leaf margin. Infected leaves will eventually turn entirely yellow and then fall off.

Cultural controls include making sure any seed that is planted is disease-free by using hot-water seed treatments. A three-year rotation out of cole crops also is advised because the bacteria is known to overwinter in the soil for more than two years.

According to Smart, some varieties are more tolerant than others, so varietal selection also can help control black rot.

Copper products can be used to help reduce the spread of the bacterium from plant to plant. However, because the bacterium can move systemically within the xylem or veins of the plant, copper is not effective once a plant is infected.

Smart’s lab has been comparing the genomic fingerprints of the isolates of X. campestris pv. campestris, and has found that new isolates enter New York each year.

“While a strain can survive on a farm from one season to the next, it is more common for new isolates to be collected each year. This is helpful in management because it is critical to know the source of inoculum when trying to manage the disease,” Smart says.

If the inoculum is coming from on-farm sources, sanitation practices will be important to rid the farm of the bacterium. However, if the bacterium is arriving on seed in transplants, then control strategies can be used during transplant production.

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