STUDIES ON THE EPIDEMIOLOGY OF STRIPE RUST OF WHEAT IN NORTH CHINA

S.M.CHEN,C.P.CHOU,S.P.LEE,K.N.WANG,Y.OU-YANG,S.W.HUNG,(North China Agricultural Research Institute) S.I.LU,T.M.YANG,W.C.WU(Institute of Applied Mycology,Acadamia Sinica)

1. In this study which has been carried on since 1949,attempts were made toanalyse factors involved in the outbreaks of stripe rust of wheat in North Chinawith a view of giving some clues for forcasting.Five periods of the developmentof an epidemic of the stripe rust in this region are recognized:(1) the infectionand divelopment period of the rust on wheat seedlings in the autumn,(2) the over-wintering period,(3) the over-springing period,(4) the development period in thelate spring,and (5) the over-summering period.For each of the above stated peri-ods,the degrees of the stipe rust development are described and the main factorseffecting the development are analysed. 2. In the autumn,if field conditions were favorable for infection,the earlier ??the date of sowing,generally the heavier the wheat crops would be rusted.Therewere little or no stripe rust developed when the date of sowing wes later than thelast part of September although the condition might be varied appreciably withdifferent localities.Besides temperature,the period of high concentration of air-bornespores was the most important factor determining the disease severity in the earlysown wheat fields. 3. The stripe rust organism was capable of over-wintering in the form of dormant mycelium within infected leaves under the weather conditions of North Chinaplain.Heavy infection of wheat seedlings was the prerequisite of over-wintering,which was also influenced by micro-environmental conditions such as air temperat-ure,soil moisture and protection from wind.Air temperature was probably the mostessential factor. 4. The dry weather and the low moisture contents in the soil in the earlyspring were unfavorable to the development of the rust.Under these conditions,the over-wintering rust might be completely killed off or much lessened,while hig-her soil humidity usually favored the overspringing of the rust organism.Furtherdevelopment of the disease appeared to depend on the amount and frequency ofprecipitation.However,in flooded areas where the soil moisture was excessivelyhigh,epidemics might occur under dry weather. 5. The over-wintering inoculum was most important for local epidemics in spring,although long distance air-borne spore might also have some influence on the rustdevelopment in the later part of the wheat growing season. 6. Uredospores of stripe rust were not capable of over-summering on volunteerwheats in North China plain.Stripe rust collections from Elymus chinense andAgropyron spp.were capable to infect certain wheat varietes when inoculated arti-ficially,but under natural conditions these grasses appeared to play little part inthe aestivation of this rust in the vast wheat growing areas of North China.Theresults of spore-trapping indicated that Chang-Chia-Kow region and certain partsof inner Mongolia where spring wheats mature as late as the end of August weremost probably the reservoir of over-summering fungus which serve as the mainsource of autumnal infection. 7. Since there are great diversity of types of farming in North China,i.e.,(1)the dry land,(2) the well and river-irrigated land,(3) the low land and river-irrigated land of the mid-part of Shansi Province,and (4) the autumn floodedland,and as the environmental factors effecting the first four stages of stripe rustdevelopment were varied,consequently,the rust development in each type of landwas obviously different from others.The disease struck most severely in the wellor river-irrigated and the autumn-flooded lands. 8. Based on the foregoing results,the writers suggest that by means of timelyobservations and inspections on the stripe rust development during autumn and onthe amounts of the Survival after over-wintering and over-springing along with theweather forecasting data,it is possible to make fairly accurate forecast of the striperust epidemics in North China.