Fungicidal Drenches for Control of Postharvest Decay in Pome Fruit
Blue and gray molds are the most common postharvest diseases of pears and apples. Gray mold, caused by Botrytis cinerea Pers.: Fr., is the most frequently encountered disease in untreated fruit stored in bins. Because B. cinerea frequently spreads from fruit to fruit (nests), losses from initial infection foci can be large. Furthermore, fruits with undetected nascent infections often are packed and can lead to situations in which boxes of fruit must be repacked before shipping. Preliminary studies have shown that both primary and secondary gray mold infections can be prevented by treating fruit with fungicides immediately after harvest and before it is put away in storage.
Currently, fungicide drenches are not used commonly on winter pears because outbreaks of blue mold and Mucor rot (caused by Mucor piriformis Fischer) have occurred in the past and been attributed to their use. Drench mixtures are typically applied directly to fruit in field bins upon receipt at the warehouse and the mixture is collected and recirculated. Postharvest drench mixtures can be a source of inocula for postharvest diseases. Large numbers of propagules of several Penicillium species, including P. expansum Link, and P. solitum Westling, were recovered from recirculating pear drench mixtures in the Mid-Columbia area (Sanderson and Spotts, 1995). Rates of pathogen inocula loading in drenches have not been established.
Dose/response relationships have been determined for conidia of P. expansum on McIntosh apple (Blanpied and Purnasiri, 1968) and P. expansum and M. piriformis on Anjou fruit (Spotts, 1986). These relationships were developed using inoculum of each pathogen alone suspended in fresh water. Therefore, possible interactions with other filamentous fungi, yeast, or bacteria, which are abundant in drench mixtures and may exert some antagonistic effects, would not have been accounted for.
The following studies were undertaken to determine if chemical and biological fungicide drenches could be used safely to control gray mold without incurring outbreaks of blue mold or Mucor rot, to model spore loading in drench systems, and to determine dose/response relationships using commercial drench mixtures.
Materials & Methods
Treatments (listed in Table 1) were applied in a randomized complete block design to bins of commercially harvested d'Anjou pear fruit in 1996 and 1997. In addition, a treatment of chlorine alone followed by a fresh water rinse was included in 1997. Fruit bins were loaded onto straddle carriers (18 to 24 bins) as they were harvested from each of 10 grower lots and treatments were applied with a drive-through, recirculating, cascading drencher (2000 gal) for 1 min. Each grower lot constituted a block and each straddle carrier load constituted a single replicate. Bins of fruit from each grower were drenched in the order in which they were received at the warehouse. A single tank was filled with each drench suspension and all replicates were treated with the same batch of suspension. A total of 400 to 600 bins of fruit were treated with each tank of drench suspension, at which point the pumps could not pick it up and the remaining suspension was discarded (about 500 gal). Most of the bins of fruit included in the experiment were treated within the first 250 to 300 bins drenched.
After treatment, all bins of fruit were placed in controlled atmosphere (CA) storage. After 5 months of storage, treated fruits were packed on a commercial packing line. All fruits with decay symptoms were removed at a presorting table as they left the dump tank and the type of decay (e.g., gray mold, blue mold, etc.) determined. Number of fruit per bin with each disease was calculated and log transformed data were analyzed with ANOVA (Minitab, State College, PA 16801). Comparisons among means were made with Tukey's HSD.
To determine the rate at which spores of species of Penicillium accumulated in drenches, samples from eight DPA/TBZ drench tanks at seven warehouses were collected and inoculum density assessed. Initial samples were taken just after a batch was made up and every 100 to 200 bins thereafter until it was disposed of.
Drench samples were diluted and cultured on Czapeks Yeast Agar amended with 200 ppm chloramphenicol. Colonies of P. expansum, other Penicillium species, M. piriformis and other potential pathogens were enumerated after 5 to 7 days of incubation at room temperature. Data were analyzed by regression of square root transformed numbers of colony forming units (cfu)/mL of Penicillium spp. vs bins/100 gal tank capacity (Minitab, State College, PA 16801).
Wounded Red Delicious fruit were dipped in drench tank mixtures removed from six commercial drenchers in 10 drencher runs at about 250-bin intervals. Three replicate fruit lots of 30 fruit each were wounded (two wounds/fruit) with a blunt nail to simulate stem punctures (5 mm diameter, 3 mm deep) and dipped in the drench mixtures. Fruit was then placed in trays in boxes and stored in regular atmosphere storage. The proportion of fruit with lesions was determined about 90 days after treatment. Inoculum density present in each drench mixture at the time fruit were treated was assessed as described above. Data were analyzed by regression of blue mold incidence vs cfu/mL of Penicillium spp. (Minitab, State College, PA 16801).
Results & Discussion
Gray mold. In untreated fruit, the incidence of gray mold was higher than that of any other disease in both years (Table 1). All drench treatments significantly reduced disease incidence over that in the untreated controls. In 1996, the incidence of disease observed in fruit treated with Biosave 110 + Mertect 340-F (TBZ) was higher than that observed in the fruit treated with Mertect 340-F alone even though the full rate of Mertect 340-F was used. It is not clear why this interaction would occur. In 1997, the rate of Mertect 340-F mixed with Biosave 110 was dropped to about one-third of the label rate and the level of disease control achieved was equal to that observed in fruit treated with Decco I-182 + Mertect 340-F (also at one-third rate).
Blue mold. In these trials, decay caused by any species of Penicillium was classed as blue mold. The majority of blue mold was caused by P. solitum rather than P. expansum. Blue mold incidence in most treatments was about a third higher in 1997 than in 1996 (Table 1). In both years, blue mold incidence in fruit treated with Biosave 110 + Mertect 340-F was not significantly different from that in the untreated controls. In 1996, blue mold incidence in the Captan + TBZ treated fruit was equal to that in the untreated control, but in 1997 it was significantly less than that in the control. The addition of Decco I-182 did not significantly affect the incidence of blue mold from that seen in the treatment with Mertect 340-F alone in either year.
Mucor rot. Mucor rot incidence in all treatments was low in both years (Table 1). The absence of Mucor rot in treated fruit in 1996 probably should not be taken as an indication that the treatment lessened the incidence of disease as much as its presence indicated a lack of control. Mucor rot was observed in all treatments in 1997. While decay incidence was not significantly different than that observed in the untreated control, significantly less Mucor rot was observed in the Captec 4L + Mertect 340-F treatment than in treatments with either of the biological antagonists. Of these, only Biosave 110 claims efficacy against Mucor rot, which was not evident in these trials.
Other decay. This category includes decay caused by Alternaria rot, Cladisporium rot, Coprinus rot, and bull's-eye rot. The former two diseases were primarily in large wounds that originated in the field (e.g., bird pecks and insect feeding wounds), while the latter two frequently were not associated with visible wounds. All treatments except Decco I-182 + Mertect 340-F reduced the incidence of these latter diseases over that observed in the untreated control.
Total decay. The total amount of decay/bin shown in Table 1 is the sum of disease incidence from gray mold, blue mold, and Mucor rot in 1996, and from those and "other decay" in 1997. The greatest reduction in decay in both years was observed in the captan + Mertect 340F treatments. Captec 4L may be more effective than Captan 50WP for this application but no direct comparison of the two formulations was made.
In both years, disease incidence in fruit treated with either biological antagonist + TBZ was not significantly different than that in fruit treated with Mertect 340-F alone. Fruit treated with a pre-rinse of chlorine followed by Mertect 340-F had significantly less decay per bin than did those treated with Mertect 340-F alone. However, fruit treated with chlorine alone were burned and were not packable, therefore no decay assessments were made on those fruit.
Commercial drenching systems were of two types, static volume drenchers and astatic volume drenchers. Static volume drenchers were characterized as those in which the volume of the drench mixture in the holding tank was maintained at a given level by refilling the tank with water and chemicals regularly as the volume was depleted during use. In these systems, tank mixtures were discarded either when a preset number of bins had been treated or the operator arbitrarily decided that the mixture was too dirty to be used. In astatic volume systems the drench mixture was used up as fruit was treated and not replaced. Typically in astatic systems, two tanks were used in succession and the remaining material in each tank was then mixed. The residual was discarded when it could no longer be picked up by the pumps.
High populations of yeasts and bacteria were recovered from drench samples. After several hundred bins had been treated, populations of these yeasts and bacteria clearly inhibited the growth of slower growing filamentous fungi such as species of Penicillium. Therefore, estimates of population densities of Penicillium spp. must be considered as relative population estimates after about 500 to 600 bins treated.
The astatic volume systems became loaded with spores of Penicillium spp. at a significantly higher rate (P < 0.001) than did the static volume systems (Figure 1). Consequently, higher numbers of colonies were recovered after a given number of bins treated/100 gal tank capacity in astatic volume systems than in static volume ones. The current labels for both DPA and Mertect 340-F recommend treating no more than 30 bins/100gal of drench mixture. Therefore, based on these results, drenching at the recommended number of bins/100 gal of drench mixture would result in the accumulation of about 1,364 cfu/mL and 3,026 cfu/mL in static volume and astatic volume systems, respectively. In addition, colonies of Penicillium spp. were recovered even from fresh batches of drench mixtures in both systems, which indicates that tanks were not thoroughly cleaned between batches.
Individual fruits were counted as decayed when either of the two wounds made on the fruit showed symptoms. In most cases, however, when a fruit was decayed lesions developed in only one of the two wounds. Up to 5% of fruit became diseased even when no colonies of Penicillium spp. were recovered from the drench mixture. Fruit used in this study were removed from field bins just before they were wounded and dipped and were not surface disinfested, therefore, some contamination of fruit wounds could be expected either from spores in the air or on fruit surfaces.
Blue mold incidence increased linearly in proportion to inoculum dose (Figure 2). About 50% of fruit (25% of wounds) become infected when dipped into a drench mixture with about 1.4 ´ 103 cfu/mL of Penicillium spp. In the drench systems sampled, P. solitum and species of Penicillium other than P. expansum were recovered at higher frequencies and densities than was P. expansum. Furthermore, it is likely that interactions occurred among yeasts, bacteria, and the decay fungi that accumulated in the drench mixtures.
In laboratory studies, P. expansum has been shown to be more a virulent pathogen than are these other species of Penicillium (Sanderson and Spotts, 1995). The dose/response rate observed in this study was about half that found in inoculations of wounded fruit with conidia of P. expansum alone suspended in fresh water. On newly harvested McIntosh apples, the incidence of decay increased rapidly to about 50% of wounds infected at an inoculum density of 2.0 ´ 103 conidia/mL and more slowly after that to about 90% of wounds infected with 1.5 ´ 104 conidia/mL (Blanpied and Purnasiri, 1968). Anjou pear fruit appear to be more susceptible to infection than apple fruit. About 75% of wounds in similarly inoculated Anjou fruit developed symptoms when inoculated with 1.5 ´ 103 P. expansum conidia/mL (Spotts, 1986).
Extrapolating from these data, the estimated risks of drenching with recirculating drenches can be ascertained. The estimated percentage of wounds that are likely to become diseased at a given inoculum dose can be determined from Figure 2. The number of bins that can be drenched before a particular inoculum load will build in a typical drencher is shown in Figure 1. Using these figures an operator can determine how to use a drench given a particular level of risk.
Drenching with fungicides appears promising for controlling decay of pear fruits stored in bins. These data show that drenching with fungicides can be effective for reducing decay losses from gray mold and other decays (i.e. bull's-eye rot). However, in most cases the incidence of blue mold and Mucor rot was higher in drenched fruit than in undrenched fruit, although overall decay was reduced. Within the parameters of these studies, all drenches reduced the number of fruits decayed from that in the untreated controls. The greatest reduction in decay was observed in fruit treated with captan + TBZ. Great care must be taken using recirculating fungicidal drenches because of pathogen spore loading. If fruit bins carrying organic debris are treated, debris will be washed into the system and inoculum loading will be accelerated. Any efforts to extend the use of drench mixtures will increase the risk of blue mold and Mucor rot.
Blanpied, G. D., and Purnasiri, A. 1968. Penicillium and Botrytis rot of McIntosh apples handled in water, Plant Dis. Rep., 52: 865-867.
Sanderson, P. G., and Spotts, R. A. 1995. Postharvest decay of winter pear and apple fruit caused by species of Penicillium. Phytopathology 85:103-110.
Spotts, R. A. 1986. Relationships between inoculum concentrations of three decay fungi and pear fruit decay, Plant Dis., 70: 386-389.
Dr. Peter G. Sanderson1, Ken Miller2, and Diane L. Bennett1
1Washington Tree Fruit Research Commission
2Blue Star Growers, Inc.,
14th Annual Postharvest Conference, Yakima, Washington
March 10-11, 1998