Research report from OSU's North Willamette Agricultural Research and Extension Center
Delbert Hemphill
OSU Dept of Horticulture, NWREC
Introduction
Willamette Valley spinach and lettuce growers have complained of a yellows disorder, or chlorosis, affecting these crops. The bright marginal yellowing is particularly severe in spinach and in some lettuce cultivars, most notably, the Oregon State University crisphead release, 'Summertime.' Symptoms are usually less pronouced in other crisphead cultivars such as 'Ithaca' and 'Salinas,' but can be quite pronouced in romaine types. Red leaf lettuces do not usually show strong symptoms. Similar symptoms have been observed in Chinese cabbage cultivars.
Research carried at NWREC in 1987 eliminated soil pH, nutrition, and Fusarium wilt as causes of the symptoms. Cucumber mosaic virus and beet western yellows virus (BWYV) were isolated from plant tissue samples by the OSU Plant Disease Clinic. After severe outbreaks of the disorder in 1988, we decided to sample grower fields during the 1989 season to confirm the identity of the virus associated with the symptoms and to determine the severity of the problem.
Methods
Samples were collected from three grower cooperators in the Aurora, Milwaukie, and Parkrose areas of the Willamette Valley. At the first sampling date, on July 18, samples of romaine lettuce were collected from the Aurora site and from the Milwaukie site, and samples of 'Salinas' crisphead lettuce were collected at the Aurora site. Symptoms were not observed on spinach at Aurora, or on lettuce at the Parkrose site on this date. For each crop and site, a single leaf was taken from each of two plants judged asymptomatic and from four plants showing the typical marginal chlorosis, but thought to be free of other diseases or disorders.
For the second sampling, on August 29, spinach was collected at Aurora, and romaine and crisphead lettuces at Aurora, Milwaukie, and Parkrose. Samples were subjected to enzyme-linked immunosorbent assay (ELISA) with an extremely sensitive beet western yellows virus monoclonal antiserum, capable of detecting extremely low (less than 5 ng/ml) BWYV concentrations.
Results
At the first sampling date, all plants thought to be free of symptoms were found to be free of the virus, at least at the detection limit of the ELISA. Of the 12 samples identified as symptomatic, seven contained high levels of the virus and five were virus-free. Symptoms were apparently more clear-cut in 'Salinas,' as all six samples were judged correctly. ELISA results agreed with visual observations of syptoms in only seven of 12 instances for romaine. Field observations of symptoms indicated a higher degree of disease incidence at the Aurora site than at the Milwaukie site. Virus incidence was visually estimated at 4 percent for romaine at Aurora, 25 percent for 'Salinas' at Aurora, and less than 1 percent for romaine at Milwaukie.
At the second planting date, only two of 14 spinach samples did not contain BWYV, according to ELISA, even though six of the 14 samples were judged free of symptoms. The BWYV concentration in spinach was consistently 10- to 50-fold higher in spinach than in lettuce, indicating a higher tolerance to the disease in spinach. All 10 lettuce plants collected as disease-free controls were free of detectable BWYV. Plants judged to have the yellows disorder contained very low concentrations of BWYV. It is not known whether these lettuce cultivars resist the synthesis of BWYV or had only recently been infected. Disease incidence was judged to be high (over 50 percent) at all three locations, for both crisphead and romaine lettuce.
The ELISA data were of very high quality, with replications of the same sample always in close agreement. ELISA should be a dependable tool in identifying outbreaks of this virus.