Assessment of inoculation techniques to evalute apple resistance to Phytophthora cactorum

[摘要] ENGLISH ABSTRACT:Phytophthora cactorum (Lebert & Cohn) Schrot. is the primary cause of crown, collar androot rot diseases of apple (Malus domestica Borkh.) trees worldwide. This pathogen is mostdestructive in commercial apple orchards under waterlogged soil conditions and has recentlybeen identified as causing serious disease in some South African apple orchards. Crown,collar and root diseases are difficult to control because of their unpredictability andcatastrophic nature. The use of resistant cultivars and rootstocks is economical andenvironmentally considerate. Therefore the need to develop screening techniques that willenable the selection of desirable disease resistant traits as part of an apple-breeding program inSouth Africa was identified. The work undertaken in this study was aimed at optimizingdifferent techniques to test resistance.Using two direct inoculation techniques (excised stem and intact stem) theaggressiveness of lO isolates of P. cactorum on apple rootstocks was determined. Thesusceptibilities of five apple rootstocks were also compared. Results have shown isolate byrootstock interaction which means isolate aggressiveness was influenced by rootstocks tested.The selectivity of isolates suggests that there may be several strains of the pathogen.Population studies of the pathogen might contribute valuable information that could lead tobetter interpretation of results. Rootstock susceptibility was monitored in vitro throughout theseason by inoculating at monthly intervals for 26-months. It was observed that during winter,rootstock susceptibility was low compared to high susceptibility during summer. Theseresults have revealed new information regarding changes in the relative resistance of thedifferent rootstocks over the growing season, e.g. the susceptibility pattern of rootstockMMl06 occurred 1 to -2 months later than that of other rootstocks. This finding hasimportant implications on the way in which resistance test results are interpreted, andemphasizes the importance of not relying on point sampling. Furthermore, useful informationhas been acquired regarding the epidemiology of the disease with regard to windows of susceptibility. The phenomenon of a phase shift in susceptibility of different rootstocksneeds to be tested on a broader scale to assess whether it has any practical application onresistance testing.Although different inoculation techniques are applied in breeding programs, up to nowthere is no consensus on which technique works best for seedling selections. Since largenumbers of individuals must be tested to improve the chances of detecting resistant genotypes,mass inoculations of young seedlings is a rapid way of identifying resistant individuals. Twodifferent screening methods were tested during this study. Using the sand-bran technique,seedlings were transplanted onto inoculated soil and the root mass was used as a measure ofresistance. In a second method zoospore inoculum was applied to seedlings growing in asand:bark mixture at different concentrations and the seedlings were subjected either to waterdrenching or not. In both trials the aggressiveness of isolates differed significantly from eachother and only higher inoculum concentrations were effective in causing disease. The age ofseedlings used in tests emerged as an important factor. Seedlings under five-months-oldshould not be used. Drenching inoculated seedlings enhanced disease development but theproduction of sufficiently high numbers of zoospores was a laborious task. Thus, it isrecommended that the sand-bran inoculum technique be tested with the drenching treatmentfor mass selection.In conclusion this study confirms the importance of both choice of isolate and choice ofinoculation intervals in determining susceptibility of rootstocks to infection. In spite of thefact that stem inoculation bioassays have limited resemblance to natural disease situations,these bioassays are useful for obtaining an indication as to whether genotypes have a degree ofresistance and merit further testing. For this reason refinement of the stem inoculationbioassay is worthwhile pursuing. With regard to seedling trials, both the sand-bran and thezoospore technique appear promising but refinement of these techniques is necessary in orderto present a more practical way of testing large volumes of seedlings.

[发布机构] Stellenbosch University