A study on the etiology and epidemiology of black root rot of strawberries in the Western Cape

[摘要] ENGLISH ABSTRACT:Historically root diseases have been a production-limiting problem for the strawberryindustry worldwide. In the Western Cape Province of South Africa the most seriousroot disease is black root rot, which causes losses of up to 30%, annually. The aimsof this study were to investigate aspects of the etiology and epidemiology of thisdisease in the Western Cape, and to provide information that can be incorporated in anintegrated disease management strategy. In Chapter I a summary of publishedinformation on this disease is presented. Disease symptoms include severe stunting ofplants, which have black, rotted, reduced root systems. Even though this disease is ofgreat economic importance, the etiology remains unresolved. However, soilbornefungal root pathogens, particularly Pythium and Rhizoctonia spp. have beenimplicated as major role players. Control of this disease, as well as the other rootdiseases affecting strawberries, has relied on soil fumigation with broad spectrumchemical fumigants, in particular methyl bromide. However, due to the ozonedepleting action of methyl bromide it was decided at the signing of the MontrealProtocol to remove this chemical from the market. This action has caused greatdemand for alternative measures to control root diseases on many crops includingstrawberries. Development of integrated disease management strategies is dependentupon a more complete understanding of the etiology, biology and ecology of thedisease complex.In Chapter 2 fungal pathogens associated with diseased plants were isolatedand Koch's postulates were carried out. The most frequently isolated fungalpathogens were Pythium irregulare, Rhizoctonia spp. and Cylindrocarpondestructans. Two morphotypes of Rhizoctonia were isolated viz. a brown and a whitetype. Pythium irregulare was isolated more frequently in July than in September, andwas not isolated at all in November. Rhizoctonia spp. were present at all samplingdates but were more frequently isolated in November than at the other times. All thefungi that were tested were pathogenic and caused root lesions. Cylindrocarpondestructans and Coniella fragariae did not have a stunting effect on the plants. Theseresults confirm a major role for Pythium and Rhizoctonia in the black root rotcomplex and suggest that there is a complimentary seasonal variation in occurrencebetween these two pathogens.The Rhizoctonia species and anastomosis groups of isolates obtained fromdiseased strawberries in the Western Cape Province were determined, and theirpathogenicity and relative virulence assessed. Both binucleate and multinucleatetypes were recovered from diseased roots and identified as R. fragariae and R. so/ani,respectively. All isolates of R. solani were members of anastomosis group (AG) 6,whereas three AG types were identified among isolates of R. fragariae, viz. AG-A,AG-G and AG-I at a relative occurrence of 69%, 25%, 6% respectively. AllRhizoctonia isolates were pathogenic to strawberry, but R. solani (AG 6) was the mostvirulent causing severe stunting of plants. This is the first species confirmation andAG type identification of Rhizoctonia taxa causing root rot of strawberries in SouthAfrica.An assessment of the presence and quantity of black root rot pathogensassociated with soils prior to fumigation and post fumigation with methyl bromide,was made in Chapter 4. Isolations were also made from nursery plants to determinewhether any black root rot pathogens were in the plants before transplanting. Resultsdemonstrated that after fumigation the soil was free of all pathogenic fungi associatedwith the disease. However, the main pathogens involved in black root rot, viz.Rhizoctonia fragariae, R. solani and Pythium spp. were isolated from nursery plants.The fact that the plants are already infected with these pathogens renders the prospectsfor control of this disease difficult. Further studies are urgently required to developproduction practices that can be included in disease management programmes.In vitro studies were carried out to determine the ECso values of differentfungicides for isolates of Pythium irregulare, Rhizoctonia fragariae AG-A, AG-G andAG-I and R. solani AG 6. Benomyl, fludioxonil and tolc1ofos-methyl were used inthese tests. Field trials were also conducted using these fungicides. In Chapter 5 it isshown that in general application of fungicides improved the yield and did not affectthe survival rate of strawberry plants. Fludioxonil showed potential for short-termuse. Applications of fungicides that targeted specific fungal genera were notsufficient to control the disease. Seasonal fluctuation of Pythium and Rhizoctoniaspp. became apparent with the occurrence of Pythium being relatively high early inthe season but low late in the season. Conversely, the occurrence of Rhizoctonia waslow at the beginning of the season but high late in the season. In the short-term thereis potential for fungicide applications as part of an integrated disease managementstrategy, but the economic feasibility of this practice needs to be assessed.In this study the major pathogens causing black root rot were identified in theWestern Cape Province of South Africa, and important information regarding theepidemiology of the disease was reported. These results can be incorporated in anintegrated management strategy to reduce losses of strawberry production, attJibutableto black root rot.

[发布机构] Stellenbosch University