Biological control of brown rot disease caused by Monilinia Laxa in cherries and plums

Monilinia laxa is the causal agent of brown rot disease on stone fruits, and also causes blossom wilt and twig canker. The common practice used to manage this disease is through fungicide treatments, especially during the flowering and fruiting period. However the demand to reduce fungicide input has been increasing and there is a growing number of reports of M. laxa strains that are resistant to fungicides. This study is based on the development of biological control agents (BCAs) as an alternative strategy for disease control using either existing BCA products or new indigenous isolates from UK orchards. Efficacy of exotic commercial BCAs against M. laxa was investigated using in vivo tests on cherries and plums. Serenade partially inhibited M. laxa while the other four BCA products (BioPK, BoniProtect, Prestop and Trianium) had no effect against the pathogen.

Indigenous BCAs were isolated from healthy leaves, intact fruits and mummified fruits of cherries and plums collected from orchards in Kent. A total of 217 isolates were screened against two strains of M. laxa in a series of in vitro tests using a dual culture technique. From these tests, 12 isolates were selected for further screening. The final screening based on in vivo tests on cherries and plums narrowed these down to two isolates with good potential for development. They were further tested under post-harvest conditions by dipping cherries and plums in suspensions of cells of the individual control agents and storing treated fruit under standard storage conditions. The two BCAs did not control brown rot on cherries under these conditions, and the low incidence of natural infection by M. laxa in plums also meant that significant control could not be demonstrated.

The two BCAs were identified as Bacillus amyloliquefaciens/subtilis (isolate B91) and Aureobasidium pullulans (isolate Y126). Their modes of action were investigated. Bacillus sp. B91 was shown to produce soluble and volatile organic compounds which inhibited M. laxa, while A. pullulans Y126 apparently competes with the pathogen for nutrients and did not produce inhibitory compounds. The capability of B91 and Y126 to grow and survive at low temperatures was studied. Bacillus sp. B91 was shown to be a mesophillic bacterium that could grow at 10-25°C but suffered significant mortality at 0 and 5°C, while A. pullulans Y126 was both mesophilic and psychrotolerant because it grew between 0-25°C, although 20°C was the optimum temperature. Once all nutrients were removed, Y126 was able to survive for several weeks in all test temperatures (0-25°C) but showed significant mortality at 25°C. The capability of B91 to survive at 20 and 25°C was higher than low temperatures (0-15°C).

Molecular studies were used to show that M. laxa populations on mummified fruits were likely to be responsible for fruit rot infection in the same orchards the following season. Geographical origin and host also influenced the population structure of M. laxa. Field trials were conducted in which mummified fruits were either treated with a commercial fungicide (Indar) or BCAs to investigate whether these treatments would significantly reduce sporulation of the pathogen in the field thus reducing the infection load in the spring. Spraying in Winter alone was not sufficient to significantly suppress sporulation of M. laxa but the efficacy was improved when the control agents were applied in Spring either as a single spray or in combination with a Winter spray. Sprays of Indar or A. pullulans Y126 greatly suppressed sporulation on mummified fruits if the fruits were treated on both occasions. It was concluded that the two novel, indigenous BCAs have potential to control M. laxa but more research is necessary to develop these two potential BCAs for field use.