Efficacy of Pre-harvest Fungicide Applications and Cold Storage for Postharvest Control of Botrytis Fruit Rot (Gray Mold) on Red Raspberry

Applications of the fungicides Elevate 50WG and Switch 62.5WG to red raspberry prior to harvest were evaluated for post-harvest control of fruit rot, caused by Botrytis cinerea. Fungicides were applied three times during bloom only, three times pre-harvest (post-bloom) only, or six times full season (bloom through harvest), and symptomless fruit (at harvest) were incubated for 8 days in either cold storage (4°C) or at room temperature (23°C). Without fungicide, between 6 and 29% of the fruit had rot symptoms after 2 days of storage at room temperature over the 2 years of testing; after 8 days, 74 and 85% of the fruit had symptoms at room temperature. Zero and 12% of fruit had symptoms with 2 days of cold storage when plants were not treated with fungicide, and rot incidence increased to 9 and 60% at 8 days of cold storage over the 2 years. All fungicide treatments significantly reduced post-harvest fruit rot compared to the control (six water sprays). Applications of fungicide only at bloom generally had higher fruit rot incidence than found for the pre-harvest and full season fungicide programs. Results indicate that pre-harvest (post-bloom) fungicide sprays are beneficial for control of post-harvest Botrytis fruit rot, especially when coupled with cold-temperature storage. Introduction One of the major constraints to increased raspberry production and marketing worldwide is Botrytis fruit rot or gray mold, caused by the fungus Botrytis cinerea (8,12). The raspberry is an aggregate fruit composed of about 100 drupelets. Its morphological characteristics contribute to its high susceptibility to fruit rots (9,12). The lack of effective post-harvest control of gray mold is the most important single factor limiting the sale of fruits on distant markets (9). In Ohio, almost 100% of raspberry fruits are marketed as pick-your-own or direct retail in farm markets. Growers that market harvested berries often need to hold them in cold storage for at least 2 to 3 days for marketing purposes; however, many growers experience 100% loss of fruit in cold storage within 48 to 72 h due almost exclusively to rot caused by B. cinerea. B. cinerea also causes gray mold of strawberry. Epidemiological studies have demonstrated that the majority of fruit infections on strawberry occur during bloom (2,3,14). Studies on raspberry have had similar results (4,6,18,19,20). Williamson et al. (20) reported that inoculation of flowers with dry conidia greatly reduced the shelf life of fruit after harvest. They also found that most fruits are symptomlessly infected via flower parts prior to harvest (4,5,9,10). Jarvis (10) reported that mycelium infecting floral parts can invade the proximal end of both strawberry and raspberry receptacles. This mycelium generally remains quiescent (latent) until fruit begin to ripen, at which time the fungus becomes active and rots the fruit. Even under relatively dry environments, 16 October 2008 Plant Health Progress conidia can germinate in stigmatic fluid and symptomlessly colonize styles that remain attached to fruits at maturity (4,8,12,20). Due to the importance of flower infections, fungicide applications for gray mold control on strawberry and raspberry are currently targeted primarily during the bloom period (2,3,4,8,11,17,20). On strawberry, bloom applications have been very effective for gray mold control in perennial production systems (1,9,14,17). In perennial matted row strawberry production systems, there is a limited bloom period that makes it relatively efficient to protect the blooms with fungicides. Currently in Ohio, most strawberry growers apply fungicides for Botrytis fruit rot control during bloom, and discontinue spraying after bloom through harvest. This approach has provided excellent control on strawberry (17). On raspberry, bloom occurs over a much longer period of time and considerable bloom can be present during harvest. Many Ohio growers make applications to raspberry similar to those made on strawberry during the primary bloom period and generally stop sprays as harvest approaches. Although this approach works very well on strawberry, our observations suggest that pre-harvest sprays on raspberries may be beneficial for post-harvest control of gray mold. Dashwood and Fox (4) suggested that multiple-spray programs on raspberry were essential to prevent both the early symptomless infection of fruit, as well as later surface contamination. The use of cold storage to aid in the reduction of post-harvest fruit rot is a clearly established and widely used cultural practice (7,11,13). The objective of this study was to evaluate the effects of fungicide timing and cold storage on post-harvest fruit rot caused by B. cinerea in raspberry. Fungicide Timing and Post-Harvest Storage Trials Fungicide evaluations were conducted in a 3-year-old commercial red raspberry planting at Moreland Fruit Farm, Moreland, OH, initially in 2003. Plants of the cultivar ‘Nova’ were grown in trellised rows on 3.6-m (12-ft) centers. Individual plots consisted of 4.6-m (15-ft) long sections of row. An untreated section of row was left between each treated section of row. Treatments were arranged in a completely randomized design with four replications per treatment. Fungicides were applied in 935 liters of water per ha (100 gal of water per acre) using a handgun at 1724 kPa (250 psi) pressure. All treatments were applied to runoff. The fungicides Elevate (fenhexamid) and Switch (cyprodinil plus fludioxonil) were applied in a one or two-spray alternating program, with the first application for each timing period being Elevate and the last application being Switch. Switch has provided good control of post-harvest fruit rots on other crops (16); therefore, Switch was targeted for use in the last application before harvest. Fungicide, rates and timing for each treatment in 2003 are provided in Table 1. Fruits were harvested from all treatments on three dates, 24 June, 29 June, and 4 July. At each harvest, 50 apparently healthy (symptomless) fruit (marketable fruit, red ripe) were hand harvested directly into 0.24-liter (2-pint) plastic clam shell containers. Fruits were picked carefully to avoid physical damage. Fruits were in contact with each other and all fruit could be observed without opening the container and moving the fruit. Two containers (each containing 50 fruit) were harvested from each replication, treatment, and harvest date. Containers were kept on ice in a cooler and within 1 h were immediately transported to the laboratory where one container per replication and treatment was placed on a laboratory bench at room temperature (23°C) and the other container was placed in cold storage at 4°C and 95% relative humidity. On days 2, 4, 6, and 8 of storage, the contents of each container were inspected for incidence of Botrytis fruit rot. Decayed fruit were left in the clamshell containers to replicate commercial conditions and also to prevent fruit damage and spread of inoculum that would accompany excessive handling (1). Disease incidence was recorded for each observation based on the presence of white mycelium, or presence of fungal sporulation. Visual diagnosis was confirmed by examination of sporulation with a hand lens or by plating out representative samples of aerial mycelium on acidified potato dextrose agar. 16 October 2008 Plant Health Progress Table 1. Percentage of raspberry fruits with Botrytis fruit rot (gray mold) symptoms after 8 days of post-harvest incubation at either room temperature (23°C) or cold storage (4°C) in 2003. z Means followed by the same letter within or across columns for incidence of fruit rot are not significantly different based on pair-wise comparison of leastsquares means using the least significant difference (P = 0.05). Analysis was based on the angular transformation of the proportion of visibly diseased fruit; mean transformed values were back-transformed to percentages for presentation purposes. y Controls were sprayed with water only on all application dates (full season). The entire experiment was repeated in 2004, with the same fungicide treatments and storage conditions. Fungicides, rates, and timing are presented in Table 2. Fruit were harvested from all treatments on 24 June, 29 June, and 4 July. Experimental procedures for harvesting, storage, and incubation of fruit were the same as described for 2003. Table 2. Percentage of raspberry fruits with Botrytis fruit rot (gray mold) symptoms after 8 days of post-harvest incubation at either room temperature (23°C) or cold storage (4°C) in 2004. z Means followed by the same letter within or across columns for incidence of fruit rot are not significantly different based on pair-wise comparison of leastsquares means using the least significant difference (P = 0.05). Analysis was based on the angular transformation of the proportion of visibly diseased fruit; mean transformed values were back-transformed to percentages for presentation purposes. y Controls were sprayed with water only on all application dates (full season). Treatment Spray timing Fruit rot (%) at 8 days