Contributing Author: Dr. Anthony Keinath
Black spot, also known as Alternaria leaf spot, is considered to be the most important fungal disease on organic kale in South Carolina. It tends to be more prevalent in the fall than in the spring. The primary cause is the fungus Alternaria brassicicola (AB), found worldwide on brassica crops. In South Carolina and Georgia, a second species, Alternaria japonica (AJ), also is present. AJ is more cold tolerant than AB, so it is more likely to be found on fall crops or overwintered kale than on spring crops.
In October 2021, two fields at Coastal REC were each transplanted to 14 kale cultivars representing four types of kale: curly, Tuscan (lacinato), Siberian, and Portuguese. One field was organic certified, and one was conventional. The same rates of organic and conventional fertilizer were applied to each field. Transplants were spaced 1 ft apart in single rows on a 3-ft-wide bed with drip irrigation and white-on-black plastic mulch. Leaves were cropped in early to mid-December and again in late December to early January. Leaves were sorted into healthy and diseased categories, then counted and weighed by category. The diseased category included leaves with any leaf spots on them, even very small ones.
Average yields differed by the type of kale. Summed over both harvests, Siberian kale grew the fastest and yielded the most, followed by Portuguese kale, curly kale, and finally Tuscan kale. Cultivars did not differ much within kale type, except White Russian yielded more than Red Russian and Russian Royale when the yields from both harvests were added together.
Kale type also differed in susceptibility to black spot when plants were sprayed with spores of Alternaria. Portuguese was the most susceptible and had more diseased leaves, 70%, than any other type or cultivar. Tuscan was the least susceptible, while curly and Siberian were intermediate. Curly kale cultivars didn’t differ in susceptibility at the first harvest, but when regrowth was harvested, Oldenbor had more disease (57% diseased leaves) than the other four curly kale cultivars. The red curly kale curly Roja had less disease than the four green curly kales, but it also had low yields. Among Siberian kales, Red Russian had less disease than White Russian. Toscano Tuscan kale had more disease than cultivar Lacinato.
Another potential way to increase kale yields is to grow plants in double rows rather than single rows. At Coastal REC we did this in fall 2020 by transplanting two rows of kale on a 6-ft-wide bed with drip irrigation and white-on-black plastic mulch. The popular cultivars Darkibor and Winterbor were used. One plant at the end of each plot was inoculated with spores of AJ. Three plants in the middle non-inoculated portion of each plot were harvested, and the inoculated plant plus the plant next to it were also harvested. Leaves were sorted as described above.
Yield of healthy leaves was increased for both cultivars in double-row plots compared to single-row plots. This yield increase was observed on both inoculated and on non-inoculated plants. Black spot decreased yields of Winterbor in double rows but not single rows and not with cultivar Darkibor at either spacing. Double rows did not increase disease as much as might be expected because the hours kale leaves stayed wet was similar in both row spacings: 17.3 in single and 18.8 in double. This study demonstrates the importance of type and cultivar on disease prevalence. Selecting a type based on the balance of yield potential and disease resistance is an important part of an integrated management program for Alternaria black spot in kale.
This work was in part supported by the Organic Transitions Program, National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2021-51106-35495.
