Graduate Thesis Or Dissertation
 

Irrigation water and plant density effects on the epidemiology of aerial stem rot of potatoes

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  • Field plots of cv. Russet Burbank potatoes from a single seedlot were established at four sites over two seasons (1985-86) in two locations in Oregon to determine the role of water-borne inoculum of Erwinia carotovora in plant infection. One site at each location was irrigated from a surface-water source and the other from a well-water source. Prior to planting, seed tubers and field soil were assayed for presence of Erwinia spp.. Populations of soft rot erwinias in water and on leaf surfaces, and the incidence of aerial stem rot were determined bimonthly from plant emergence until two weeks prior to regional harvest. Strains recovered from all sources were characterized biochemically, and Erwinia carotovora subsp. carotovora strains were typed serologically by Ouchterlony agar double diffusion. Populations of soft rot erwinias were consistently higher in surface water compared to well water. Populations ranged from 0 to 524 and 0 to 25 cfu/ml in surface-water sources in 1985 and 1986, respectively. Detection of soft rot erwinias in well water usually required enrichment. Populations of soft rot erwinias on potato foliage were highest in midseason after row closure. Populations in plots irrigated with surface-water and well-water were similar and ranged from 0 to 5.90 log cfu/g and from 0 to 6.22 log cfu/g fresh weight in 1985 and 1986, respectively. Final proportion of stems with aerial stem rot was consistently higher in well-water irrigated sites ranging from a low of 0.09 in the surface-water irrigated site in 1985 to a high of 0.51 in the well-water irrigated site in 1986. Over 90% of strains recovered from all sources were characterized as E. c. subsp. carotovora. About 25% of the E. c. subsp. carotovora strains recovered from all sources were identified serologically. At all but two sites over two years some of the strains recovered from water sources were the same serologically as epiphytic strains and strains recovered from diseased stems. A majority of stains isolated from diseased stems differed serologically from water serogroups at all locations. Field plots of potato cv. Russet Burbank were established at four sites in two years to determine the effect of plant density and plant spacing on development of aerial stem rot. Plant densities were 13, 26, and 52 x 10³ plants per hectare. The intermediate density had two treatments with different within-row and between-row spacing arrangements. Treatment effects were measured by onset of disease symptoms, final proportion of disease and area under the disease progress curve (AUDPC). In 1986, leaf area index was measured and throughout both seasons leaf wetness was monitored with a CR21X Campbell Scientific micrologger. Onset of aerial stem rot symptoms occurred earlier and final proportion of disease and AUDPC were greater in dense compared to sparse plantings. Larger differences in onset of disease and AUDPC occurred with a decrease in between-row spacings than in a decrease in within-row spacings. A linear regression model that used area under the leaf area index curve (AULAIC) for the four plant spacings as the independent variables accounted for 89-98% of the variation in AUDPC. Highest leaf area index values and longest average duration of leaf wetness preceded highest level of disease by two weeks.
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