DNA fingerprinting shows disease culprits
Shedding light on which fungal species are causing bull’s-eye rot.
New technology and DNA fingerprinting has enabled Dr. Bob Spotts to identify new species of fungi that appear to be responsible for some of the bull’s-eye rot infecting tree fruits in the Pacific Northwest. It has been known for some time that two species of Neofabraea—N. malicortis (the organism that causes anthracnose) and N. perennans (the organism that causes perennial canker)—also cause bull’s-eye rot. But Spotts’s research at Oregon State University’s Mid-Columbia Agricultural Research and Extension Center in Hood River, indicates that two other pathogens are involved. One is N. alba, and the other has yet to be named.
Rots caused by the four organisms all look the same, but DNA fingerprinting shows clear differences between the species, Spotts reported at an apple quality meeting presented by Washington State University Extension this summer.
Spotts surveyed major tree-fruit growing districts in Washington and Oregon to find out which species were causing bull’s-eye rot. In the Medford and Mid-Columbia districts of Oregon, most of the samples he collected turned out to be N. alba, whereas in Washington most were N. perennans. All the samples collected in Yakima were N. perennans, although the number of samples was relatively small.
Spotts said it’s important to know which fungicides are effective against each of the four species, and he has developed a new method to determine fungicide sensitivity.
In lab tests, N. alba was sensitive most of all to Mertect (TBZ, or thiobendazole) but also to Topsin (thiophanate-methyl), Flint (trifloxystrobin), and Scholar (fludioxonil). It did not appear sensitive to Penbotec (pyrimethanil). In contrast, N. perennans was sensitive to TBZ, Topsin, and Penbotec, but not to Flint or Scholar. Spotts concluded that TBZ was one of the better postharvest products for controlling bull’s-eye rot.
Spotts said an integrated control program is needed from the orchard to the packing house to control decay.
In a trial of preharvest fungicides on pears, Ziram gave the best control. A combination of Topsin and Nutraphos 24 gave good control of bull’s-eye rot with the added benefit of excellent control of blue and gray rot as well. It was the only treatment tested that controlled all three diseases.
The incidence of bull’s-eye rot also depends on the inherent resistance of the cultivar. Spotts has determined that Fuji is the least susceptible apple variety while Gala is the most susceptible. However, other factors, such as fruit maturity, are also involved.
Spotts said growers could reduce disease risk by not leaving fruit on the ground to feed the fungus. Spores from the decaying fruit can infect later-maturing varieties. If the fruit is left in the orchard until the following season, it can cause serious disease problems. Even dropped fruit from pollinizers could increase disease pressure in the orchard. Spotts said he is developing a way to predict the decay risk based on spore loads in the orchards.
Packing houses can also help minimize diseases by sanitizing bins and keeping drench water clean so it doesn’t inoculate the fruit with spores, he added.