Biocontrol as a key component to manage brown rot diseases on cherry

NIAB EMR: X.-M. Xu & R. Saville
University of Reading: Mike Shaw

Background

Brown rot, caused by Monilinia species, is one of most important diseases in stone fruits worldwide. In Europe, the disease is primarily caused by M. laxa whilst in North America, Asia and Australasia the main causative agent of brown rot is M. fructicola. However, M. fructicola, classified as a quarantine plant pathogen in Europe, has recently been reported in several European countries. A third species, M. fructigena also causes brown rot disease but infects fruit only via wounds unlike M. laxa and M. fructicola which can infect flowers and both healthy and wounded fruit. The life cycle of M. laxa comprises three key stages: (1) early infections causing blossom blight and twig cankers, (2) infection of fruit, and (3) survival over the winter on mummified fruits and twig cankers. Fruit is most susceptible to infection by M. laxa close to maturity. There is a growing trend to cover cherry crops after flowering; this reduces fruit splitting due to rain, one of main entry sites for Monilinia spp.

Given the importance of mummified fruit as primary inoculum, eliminating mummified fruit is one of the most important management strategies, but seldom practiced. Spraying mummified fruit with fungicides can control the disease, but it selects for fungicide-insensitive strains. Furthermore, due to EU regulations, the number of active ingredients available for commercial use has been declining rapidly. Physical removal of blighted twigs and mummified fruits is widely recommended, but is labour-intensive and may be unaffordable. During harvesting, growers are now advised to pick and discard fruit with brown rot to reduce the amount of mummified fruit overwintering on the tree. Nevertheless, in the UK, control of brown rot currently focuses on protecting flowers and immature fruits from infection by routine applications of fungicides. Despite this intensive use of fungicides, brown rot remains a significant problem in stone fruit production. Given the reduction in available fungicides and restricted usage of those available, the industry desperately needs more sustainable management methods for brown rot control.

NIAB EMR recently identified two candidate microbes that greatly reduced sporulation of M. laxa on mummified cherry and plum fruit (in the lab), and protected fruit from infection by M. laxa. These two strains were a Bacillus sp. and an Aureobasidium pullulans isolate. In controlled lab-studies they reduced brown rot development more than two commercial biocontrol products from the same species (Serenade and BoniProtect). Currently, a commercial company is formulating the two strains into biocontrol products. However, more research is necessary to understand the ecology of these biocontrol agents (BCAs) and the target disease in order to best use them.

Overall objectives

This project aims to understand microbial ecology that underpins the use of microbial BCAs to manage on stone fruit; to develop new biocontrol-based strategies for managing and to evaluate such strategies in small-scale and commercial-scale studies. Specifically, we will try to minimise overwintering inoculum and protect flowers from infection by . We expect that better control of the early epidemic stage will greatly reduce the disease as fruit matures. This work contributes to sustainable food production, by increasing yield per unit of land and carbon and by reducing post-harvest waste.

Approaches

We shall use molecular methods to study microbial survival and reproduction  a qPCR method based on the technology of the DNA-modifying dye propidium monoazide (PMA”) will be used to quantify the amount of viable microbial inoculum from environmental samples collected from the key life cycle stages. Next generation sequencing (NGS) technology will be used to study the survival of BCAs and biocontrol efficacy in relation to microbiota on the surfaces of mummified fruit, flower and developing fruit. The project will focus on four BCAs – two from NIAB EMR (currently under formulation) and the two commercial products (Serenade and BoniProtect).

Experiments will be designed to answer the following specific questions for each BCA:

  1. To what extent do BCAs survive in winter on the surface of mummified fruit remaining on the tree or on the ground?
  2. Does increasing the interacting time between BCAs and M. laxa (mummified fruit or canker) lead to increased biocontrol efficacy?
  3. Are BCAs able to reduce sporulation of M. laxa cankers as well as mummified fruit?

Answers to these three questions will determine (a) whether targeting BCAs against mummified fruit and/or cankers is an effective means to reduce primary inoculum, and (b) what is the best time for applying BCAs against overwintering inoculum.

  1. Could BCAs significantly reduce blossom wilt?
  2. Could BCAs establish on flower tissues, and then colonise surfaces of developing fruit to reduce infection by M. laxa?
  3. Does a successful control of blossom wilt, together with reduction in primary inoculum, significantly reduce infection of fruit near harvest?

Answers to these three questions will determine (a) whether BCAs are effective in reducing blossom wilt and subsequent infection of young fruitlets, and (b) what is the best time to apply BCAs for protecting blossom and fruit.

  1. Could BCAs be used together to improve biocontrol efficacy?
  2. Are these BCAs compatible with commercial fungicides used in stone fruit?

Answers to these two questions will determine how BCAs could be used in practice.

Anticipated outcomes

The research outcome will assist in the development of management strategies for brown rot on stone fruit, integrating BCAs with other management practices based on our understanding of ecological characteristics of available BCAs.

The student will be encouraged to set the specific questions above in a holistic ecosystem viewpoint. They will learn how to use molecular and statistical methods to answer ecological questions regarding the fate of microbial organisms in the natural environment, how to frame practical disease management problems into ecological questions, and how to conduct field experiments to evaluate disease management strategies. By appropriate periods spent with the industrial partners they will develop understanding of how these products would fit within the community of enterprises leading from producer to consumer.