Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/13/2009
Publication Date: N/A
Citation: Interpretive Summary: ‘Redglobe’ table grapes are a popular cultivar in California. They suffer from a postharvest disorder where by the grapes dissolve into a jelly-like mass during storage long after harvest. We applied a variety of conventional, organic, and ‘reduced-risk’ fungicides to these grapes before harvest to see if we could reduce the prevalence of this disorder.Several regimes reduced the prevalence of the disorder by about 50%, which is sufficient to warrant their use commercially to manage this problem.
Technical Abstract: Postharvest sour rot of ‘Redglobe’ grapes, also called “non-Botrytis slip skin”, “breakdown disorder”, “soft tissue breakdown”, or “melting decay” has affected this cultivar worldwide. The disorder causes berries to discolor, split, lose internal structure, and decay from veraison to harvest (Cameron at al. 1997) and later in storage. Symptoms were caused by several yeasts and bacteria (Crisosto 2001, Morgan and Michailides 2004, and Palou et al. 2005), some of which are resistant to SO2 fumigation (Palou et al. 2005). Symptoms develop more rapidly under warm conditions (in several days) than during cold storage (about one month). Organisms isolated differed among the investigators, but all are common microbes found in vineyards and they rarely infect other cultivars. We evaluated the effectiveness of preharvest applications of registered fungicides on preharvest bunch rot and postharvest sour rot of ‘Redglobe’ grapes. Experiments were conducted in a commercial vineyard near Delano. The canopy of this vineyard was not dense and the clusters were relatively exposed. It was approximately 20 years old, bilateral cordon trained, spur pruned, and drip irrigated. The vines were girdled and treated with gibberellic acid, so the berry size was large (10-12 g). Each replicate plot had 6 vines. Treatments were applied to 4 replicate plots in 2006 and to 5 replicate plots in 2007. Fungicides were applied at maximum recommended rates and water volume of 200 gallons per acre by an air-blast sprayer. All except the JMS Stylet oil contained Latron B1956 at 4 oz/100 gal. In 2006, 1 or 2 fungicide applications were done; one application 2 wk before harvest, or two applications, prior to bunch closure and 2 wk before harvest. Three boxes were filled from each plot. In 2007, two identical complete block design experiments, evaluated the effectiveness of 5 spray regimes. Bunch rot before harvest was evaluated on August 7 and August 30, 2007. Clusters were classified as having bunch rot if two or more adjacent berries were infected and dripping. At harvest three boxes were prepared from each plot, and the grapes were harvested by hand and placed into clean boxes. The boxes were stored for 5 wk in 2006 and 11 wk in 2007 at 0oC in a commercial facility with weekly SO2 fumigation to minimize interference by B. cinerea. After storage, the prevalence of sour rot and other decay was recorded. An analysis of variance was applied to arcsin of the square root of the proportion of infected berries, followed by Tukeys HSD at P = 0.05 to separate means. In 2006, Pristine, JMS Stylet-Oil, and Vangard significantly reduced bunch rot, however, postharvest sour rot was high and the treatments could not be separated at P < 0.05 (Table 1). At a P value of 0.10, however, JMS Stylet-Oil, Pristine, Vangard, or Scala reduced postharvest sour rot by about 50%. JMS Stylet-Oil left a shiny residue that was visible for about 3 wk after application. In 2007, 2 of 3 programs with Pristine, Vangard, or Scala significantly reduced preharvest bunch rot by about 50% (Table 2). Significant reductions in postharvest sour rot were caused by two applications of Scala and one of Pristine. One benefit of the fungicide programs was a reduction in summer bunch rot, from 49% among untreated clusters before harvest to as low as 22%. This is a substantial increase in yield, berry quality, and the speed of harvest, because harvesters would have less trimming to do. Control of postharvest sour rot was significant but the magnitude of reduction was not large, from 0.39% among untreated berries to 0.23% among those berries treated with the best fungicide regime. Rooney-Latham et al. (2007) reported Switch, Scala, and Pristine applications reduced postharvest sour rot, which corroborates our findings.