Graduate Thesis Or Dissertation
 

The influence of environment and pathogen variability on the infection of wheat by Puccinia striiformis West

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  • Race identification of stripe rust (Puccinia striiformis West. ) is influenced by environment, pathogen variability and host age. Isolates of stripe rust were collected in the Pacific Northwest and characterized on two sets of differential varieties; the "Oregon" and the United States. Six of the seven isolates tested on the seedling Oregon differentials were identified as separate races at the 2 C/18 C (night /day) temperature profile; however, seven races could be identified using mature plants. Eight of the 16 Oregon differentials used as seedlings gave similar infection types as mature plants with all seven isolates tested. The remaining eight varieties changed from a susceptible seedling reaction to a more resistant reaction as a mature plant with specific host-pathogen combinations. This change can be used to separate the seven isolates into races. Only the variety Leda was consistently more resistant as a mature plant than as a seedling. This "mature plant resistance" is contrasted to the "field resistance" of the variety Gaines which is conditioned by environment. Five of the eight isolates tested on the United States differential varieties could be identified as separate races. With the aid of the eight supplemental varieties all eight isolates could be separated. Six field races of stripe rust could be identified on wheat grown at 11 sites in Oregon using the Oregon field differential varieties. These varieties also indicated a shift in the make-up of the rust race populations during the last four years in the Willamette Valley. Similarities in field race characteristics at Pendleton, in eastern Oregon, and Aurora, in the Willamette Valley, were also noted. Percentage of germination and penetration of uredospores of stripe rust differed on 15 Oregon differential varieties. An unidentified factor inhibited uredospore germination on certain varieties. Penetration of stomates was also delayed on some varieties but no consistent correlation could be made with inhibition of germination or host resistance to stripe rust. Desiccation following short dew periods of three or four hours effectively reduced the amount of viable inoculum available during a 24-hour period. Inoculum removal by desiccation can be made more effective by the use of varieties that lengthen the interval from uredospore germination to penetration. Under field conditions, inoculum removal can reduce final disease severity. For example, with an apparent infection rate of 0.0146 per unit per day for stripe rust, removal of inoculum from death of germinating spores for a five day period would reduce the final disease severity by 14.7 percent over a 40-day period. Factors limiting fall spread of stripe rust in the Willamette Valley differ from those in eastern Oregon. In the Willamette Valley spore movement is by leaf to leaf contact during the wet winter months. In eastern Oregon little or no rust movement occurs until warm spring weather facilitates aerial spore movement. During most years sporulating leaves are killed by freezing winter weather and rust survival is limited to infected, non-sporulating green host tissue. Competition for infection and sporulation sites between an albino race of stripe rust and four yellow races was noted. Some mechanism inherent to the albino race and presumably other races prevents invasion and/or sporulation within previously colonized host tissue. Competition between two races of stripe rust can reduce the potential number of sites for infection on a wheat leaf by 99 percent. In addition, competition can change the ratio of one race to another in the population from 1:1 to 3:1 in one generation. Summer survival on grasses in mountain areas of Oregon is limited and does not appear to play an important role in oversummering of stripe rust in Oregon. Field race(s) of stripe rust found at the Mountain Plots appears similar to race(s) in wheat growing areas. Movement of inoculum from mountain areas to wheat fields in the fall of the year seems unlikely since rust on wheat and grasses inoculated in June could not be found the following September.
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