The occurrence of pre-harvest decay on table grapes affects yield and the subsequent development of postharvest decay during storage. Bunch rot of table grapes is associated with Botrytis cinerea, Aspergillus niger, as well as Cladosporium, Rhizopus, Penicillium and Alternaria spp. among other pathogens (Sonker et al., 2016). By Dr Pieter Louw and Dr Johan Fourie
Disease complexes like soft-tissue breakdown (STB), which include acetic acid bacteria and yeasts, can also contribute towards decay.
The risk of decay fluctuates throughout the table grape season. High-risk periods can be expected when environmental conditions favour infection and colonisation of pathogens, the inoculum/spore loads of pathogens are moderate to high and the fruit are at a susceptible phenological stage. Phenological stages that can be regarded as high risk include flowering and grape berry ripening (McClellan et al., 1973). Risk of decay is further increased if fruit is wounded.
Climate change has become a reality of modern times and can result – or has more frequently resulted – in conditions that complicate decay control. In this instance, management strategies need to be highly effective to ensure satisfactory protection of vines and grape bunches. Proactive measures are now available to negate risks (Sanzani et al., 2012). Research is required to understand the risk associated with disease development and severity of plant pathogens in South African table grape vineyards. Detection technologies are available to use as possible tools to improve management strategies and limit pre- and post-harvest decay development (Sanzani et al., 2012).
In a SATI-funded project, ExperiCo conducted a study to develop a disease management tool, specifically for B. cinerea to assist the table grape industry to identify and manage risk and subsequently reduce losses. The study included investigating vineyards in Limpopo, Northern Cape and Western Cape at different phenological stages (e.g. pre-bunch closure, véraison and harvest ready) and evaluating their condition (vineyard fl oor, canopy conditions, and crop load and condition). Grape bunches were investigated for damage, disorders and signs of decay. Disease symptoms were described and the pathogens classed to genus level. In combination with vineyard monitoring, samples (berries) were collected and transported to the ExperiCo pathology laboratory to evaluate the sensitivity of early detection methods for B. cinerea. These methods included culturing on potato dextrose agar (PDA), ONFIT (overnight freezing incubation technique) and quantitative PCR (qPCR).
Based on findings from the vineyard evaluations, wounding (specifically splits and bird damage), an overgrown (dense) canopy, a high cover crop density and substantial debris on the vineyard fl oor were potential factors contributing to decay (Figure 1). When excessive, these and other factors result in poor vineyard conditions and combined with unfavourable weather (heavy summer rainfall) during ripening, lead to high decay levels (Figure 2). Failure to negate the problem early enough or using inadequate control measures, especially in such conditions, could have devastating effects on the harvest. Depending on the cultivar, decay risk tends to be higher during bunch set and increases during the last three weeks prior to harvest. No conclusion could be drawn between production regions due to variances in cultural and management practices.
The ONFIT method was an easy and effective early detection method to highlight risk pertaining of Botrytis rot (Figure 3). The method was sensitive enough to detect B. cinerea in all of the problematic vineyards, but slow to deliver results. Similar to ONFIT, the plating method on PDA had a long incubation period, however, sensitivity was low. Only vineyards with high risk were identified (Figure 4). A duplex qPCR assay was successfully optimised and set up to quantify B. cinerea DNA and Vitis DNA in samples.
The qPCR method was the most rapid and sensitive detection method (Figure 5). It was able to detect B. cinerea in healthy berries from all of the vineyards where problems were identified. The ONFIT and qPCR methods can be combined in the future to provide an improved answer.
This disease risk monitoring platform has developed into a service that can now be utilised by table grape producers to assist in managing and reducing risk. The extended program delivered by ExperiCo also includes disease risk monitoring in pack-houses. The monitoring and detection methods can be adapted to determine the efficacy of spray programmes. Future developments include taking the disease risk monitoring tool to an online platform to facilitate feedback time and user convenience, and aim to include other fruit industries into the programme. Dr Pieter Louw and Dr Johan Fourie at ExperiCo can be contacted in this regard.
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