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Botrytis Bunch Rot in Commercial Washington Grape Production: Biology and Disease Management

Botrytis Bunch Rot in Commercial Washington Grape Production: Biology and Disease Management

FS046E
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Michelle Moyer, Viticulture Extension Specialist, Gary Grove, Plant Pathologist, WSU Irrigated Agriculture Research & Extension Center, Prosser
Details BBR symptoms, management recommendations, and how to deal with fungicide resistance. Includes color images of disease stages.
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 Key Information

  • Botrytis bunch rot infections can occur during bloom and/or ripening.
  • Botrytis bunch rot is favored by wet conditions during moderate (60–77°F) temperature periods.
  • Canopy manipulation that increases air circulation, light penetration, and spray penetration into the fruit zone, especially around flowering, can help reduce Botrytis bunch rot.
  • A fungicide program, when combined with canopy manipulation, is sometimes necessary to manage Botrytis bunch rot during dry growing seasons, but is essential during wet growing seasons.
  • Botrytis cinerea can quickly develop resistance to fungicides, so proper selection of materials, rates, and use patterns are critical to preventing control failures. Cultural practices that reduce disease pressure also reduce resistance development.

Introduction

Botrytis bunch rot (BBR) of grapes is caused by the fungus Botrytis cinerea. BBR symptoms are typified by a fuzzy, gray fungal growth on ripening and mature grape berries (Fig. 1). Infections often first appear as a single berry exhibiting symptoms, but under favorable weather conditions, BBR can spread throughout the cluster. While BBR is a sporadic problem in eastern Washington vineyards, and generally limited to years characterized by wet conditions during bloom and harvest, BBR has the potential to be very destructive. The potential for severe losses on a regular basis is greater in the cool and humid climate of western Washington.

Figure 1. Botrytis bunch rot is caused by the fungal pathogen Botrytis cinerea. Rot symptoms on grapes are typified by a gray fuzzy growth, berry oozing, and berry desiccation. Photos by Gary Grove.

Biology and Disease Development

In order to effectively manage BBR, it is important to understand the biology of the pathogen and epidemiology of the disease (Fig. 2). Botrytis cinerea grows on many plant species and decaying plant matter. At a practical level, there is nothing that can be done to exclude it from a vineyard. However, it is a relatively weak pathogen on grapes, infecting tissue through wounds or natural openings. Common wounds occur from hail, mechanical damage, insect feeding, splitting of grapes during ripening, and diffuse powdery mildew infections. It often infects from an established base of dead or dying tissue, making flower debris, dying stamens, flower cap scars, and other plant debris important sources of inoculum for berry infection.

Classic BBR symptoms on a cluster can result from two different infection types occurring at two different times of the season. The first is BBR resulting from early season infections that remain latent (hidden) until the end of the season. Once the fruit begins to soften and accumulate sugar, BBR can become reactivated, and rot berries. The second is BBR that results from late-season (véraison and later) infections of injured and/or ripening fruit, and subsequent spread to abutting berries within a cluster. While the latter often appears to be the more commonly manifested expression of the disease (often accompanied by insect and animal feeding, or late season rain events), latent infections can be extremely destructive, especially when not expressed until storage or in the fresh market. Often, BBR symptoms seen at the end of the season are the expression of latent infections that occurred between bloom and cluster closure.

Botrytis cinerea thrives under high humidity (90%+), and can grow and infect when temperatures are between 35 and 90°F, but thrives at temperatures between 60 and 77°F. Free moisture is also favorable for infection. Flower infection can occur with as little as 1.5 hours of continuous leaf wetness, whereas mature fruit infection requires a wetting duration of 14 hours.

Cultural Practices for Disease Management

Botrytis cinerea can grow under a wide range of conditions, but disease pressure is greatest in the cool, damp conditions created in dense grape canopies. Therefore, any form of canopy manipulation that increases sunlight penetration and air

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