Studies on Biological Control of Phytophthora Cactorum on Apple

by Alexander, Brett John

Abstract (Summary)
A glasshouse bioassay using Royal Gala apple seedlings was developed to screen a range of microorganisms for biological control of Phytophthora cactorum, the causal agent of crown, collar, and root rot of apple trees. Six hundred and twenty-four test isolates (504 fungi and 120 bacteria and actinomycetes), isolated from soil samples collected from apple orchard and non-orchard sites or obtained from other sources, were initially screened against P. cactorum in 2l glasshouse trials. One hundred and twenty-five isolates (114 fungi and 1l bacteria) were selected for their ability to prevent or reduce disease incidence compared with the controls over an 8 or 10 week trial period. The microorganisms were subsequently rescreened. Fifteen fungal isolates (Fusarium oxysporum F78; Gliocladium roseum Fl50 and F283; Microsphaeropsis sp. Fl5; Oidiodendron sp. F762; Paecilomycer sp. F46; Penicilliumsp. FIZO; Trichodenna harzianumFT94 andFl247;7. koningii F176,F463,F929, and F950; Ulocladium sp. Fl l8; and an unidentified ascomycete Fl38) were selected from a series of l2 glasshouse trials. These isolates provided 59-l00%o disease control of P. cactorum.In another experiment, all l5 fungal isolates (except G. roseum F283) provided control that was equivalent to that achieved by fosetyl-Al (Aliette 80 WP; foliar spray), while seven isolates (Microsphaeropsis sp. Fl5, Oidiodendron sp. F762, Paecilomyces sp. F46, T. harzianum F294 andF1247, and L koningii F463 and F929) provided control equivalent to that obtained by both metalaxyl + mancozeb (Ridomil MZ 72 WP; soil drench) and fosetyl-Al. Of the 1l bacterial isolates examined, Flavobacterium sp. B219 was selected. This isolate provided 8l7o disease control. The 16 promising isolates (15 fungi and one bacterium) were the focus of the remaining investigations. Four additional glasshouse studies were conducted to optimise disease control using selected isolates. The level of biocontrol was not enhanced when fungal isolates were grown on a wheat bran/sand medium for 3 or 7 days compared with 14 days. The results of trials examining biocontrol over a range of pathogen inoculum rates and inoculum concentrations of selected promising isolates were inconclusive. In another study, mixtures of isolates provided a high level of disease control (65-93Vo) of P. cactorum. However, disease control was not significantly different to that obtained by the individual isolates (82-95Vo). Flavobacterium sp. B2I9 provided the best overall control of P. cactorum on MMl06 apple rootstocks over a 14 week trial period. This bacterium provided significant control (65To) under conditions of high disease pressure. In addition, this treatment significantly increased rootstock height, total fresh weight, rootstock dry weights, and visual estimate of root mass compared with the pathogen control. Control was equivalent to that obtained by metalaxyl + I mancozeb. The ten fungal treatments examined provided a level of disease control (25-73Vo) after 14 weeks, however, only Penicillium sp. Fl20 (73Vo) was significantly diff'erent to the pathogen control. Of the fungal isolates examined, F. oxysporum F78, Oidiodendron sp. F762, and 7. harzianum F1247 consistently provided control that was equivalent to the fungicide treatment. To determine how the 16 isolates affected various stages of the life history of the pathogen, an in virro bioassay using strawberry seedlings was developed. Six fungal isolates (L harzianumFl24T;7. koningii isolates F463,F929, and F950; and G. roseum isolates Fl50 and F283) were consistently effective against specific stages of the life history of P. cactorum. These isolates affected sporangial production and zoospore transmission; 0-127o of the target seedlings were infected compared with >99Vo in the pathogen control. In the oospore production trial, these isolates significantly reduced oospore numbers from 415 to 0- 75. In a trial assessing the protection of the target seedlings, the isolates did not prevent initial infection but significantly reduced the number of oospores formed in the infected tissue (3- 49) compared with the pathogen control (>368). The remaining fungal isolates significantly reduced oospore production, however, only F. oxysporum F78, Microsphaeropsis sp. Fl5, T. koningii F176, and Ulocladium sp. Fll8 affected sporangial production and zoospore transmission. Flavobacterium sp. 8219 had no effect on P. cactorum in anv of the trials. Culture filtrates of T. harzianum F1247 and L koningii F463 prevented infection of target seedlings in the sporangial production and zoospore transmission trial, but the results were variable between pathogen isolates and assessments. Culture filtrates of T. harzianum FL247 also significantly reduced oospore production (0-10) compared with the pathogen controls (>3OZ). This isolate provided better control of pathogen isolate P19 than P28. The l6 isolates were inhibitory towards both pathogen isolates in dual culture. When plugs were removed from the interaction zone and plated onto fresh Phytophthora selective medium, the growth of the pathogen was completely inhibited by T. harzianum F1247, T. koningii isolates F176, F463, and F929. This, however, varied between experiments. All 16 isolates showed potential for biocontrol of P. cactorum. Of these, T. harzianwnFL24T, Flavobacterium sp. 8219, Oidiodendron sp. F762, and Penicillium sp. Fl20 were consistently effective against the pathogen in seedling and rootstock pot trials and warrant evaluation under field conditions. More detailed studies are needed on how the isolates provicie control and what mechanisms of action are involved. An understanding of the ecology of the potential BCAs in relation to the disease is also required.
Bibliographical Information:


School:The University of Auckland / Te Whare Wananga o Tamaki Makaurau

School Location:New Zealand

Source Type:Master's Thesis



Date of Publication:01/01/1999

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