Use fungicides judiciously
There is evidence that pear pathogens are developing resistance to new fungicides.
Pear producers have tools to manage postharvest rots, such as blue mold, but need to take steps to avoid resistance.
Pear producers now have a range of fungicides they can use to control rots in stored fruit, but the products must be used judiciously, says Dr. Chang-Lin Xiao, plant pathologist with Washington State University in Wenatchee. Already, there are signs of fungi developing resistance to products that have only been in use for a few years.
The major diseases affecting d’Anjou pears in Washington State are gray mold (Botrytis cinerea), blue mold (Penicillium expansum), phacidiopycnis rot (Phacidiopycnis pin), bull’s-eye rot (Neofabraea), and mucor rot (Mucor piriformis).
Phacidiopycnis and bull’s eye rot are diseases that come from latent fungal infections in the field. The fungi colonize the fruit in the orchard, but the symptoms don’t show up until the pears are in storage. Blue mold, gray mold, and mucor arise from infections of wounds that occur either at harvest or during postharvest handling.
The industry dealt with resistance of blue mold to Mertect (thiabendazole or TBZ) for many years, but now has other options including the postharvest fungicides Penbotec (pyramethanil) and Scholar (fluxiozonil), which are both effective against blue mold, gray mold, phacidiopycnis, and bull’s-eye rot.
The broad-spectrum fungicide Pristine was registered in 2005 for use in the orchard and is increasingly used by the pear industry as a preharvest treatment for decay control, Xiao said. When applied in the field, it provides some residual control of fungal diseases for up to a few weeks after harvest.
During the 2008–2009 season, the pear crop developed a relatively high level of gray mold, and the Botrytis fungus that causes it appears to have already developed some resistance to both active ingredients in Pristine, Xiao related during a Pear Postharvest Fruit School presented by WSU earlier this year.
In 2009, a low frequency of resistance of blue mold to Penbotec was also found. Penbotec, which has been used since the 2005 season, is still generally highly effective against blue mold, Xiao said; it does not seem to be losing efficacy. However, it’s a clear indication that a resistant population of the fungus is emerging.
“Some of the new postharvest fungicides have not been used judiciously, and that can be a problem,” Xiao said, advocating better stewardship of fungicides. “Major pathogens, like Penicillium and Botrytis, are at high risk for development of fungicide resistance. Judicious use of these fungicides is important to retain efficacy for decay control. We can’t afford a loss of efficacy. Take a lesson from TBZ resistance in the past. We don’t want to repeat the same problem.”
There are two ways that packing houses handle fresh pears. One, commonly used in the Wenatchee area, involves packing all the fruit within two months of harvest and storing it in the cartons. Decay, such as stem-end or calyx-end decay or wound-associated rots, can develop after packing, resulting in the need for repacking. An application of Pristine, Topsin (thiophanate-methyl), or ziram in the orchard within two weeks of harvest is critical, especially if the fruit is not drenched before storage. The fungicide should be applied from the ground to ensure good coverage. Topsin should not be used in the orchard if Mertect will be used in the packing house, as the two products are in the benzimidazole class of fungicides and use of Topsin could increase the population of benzimidazole-resistant fungi, which would jeopardize the efficacy of Mertect.
Sanitizing the water system and using an on-line fungicide during packing will help minimize losses from decay, Xiao said, but if the fruit is packed two to three months after harvest, the fungicide might not be effective for controlling phacidiopycnis or bull’s-eye rots because the pathogens might already have become established in the fruit.
The other handling method used involves storing pears in bins for up to several months and bringing them out of storage to pack them as the market demands. Decay can become a problem in the bins before the fruit is packed. A prestorage fungicide drench is important for fruit destined for long-term controlled-atmosphere (CA) storage, particularly if the orchard has a history of decay problems, Xiao said. “You can’t hold the fruit for several months without a fungicide.”
When selecting a postharvest fungicide, packers should rotate different modes of action (for example, Penbotec and Scholar) each year. Xiao expects as Mertect is used less, resistance of the Penicillium pathogen will drop, and it could then be used in a three-way rotation. The coverage and duration of the drench is important, he said. The truck should stop under the drench spray for at least 30 seconds to make sure the fruit is well covered.Xiao also recommends integrating other approaches to disease control, such as good cultural practices and orchard sanitation to minimize the disease inocula getting onto the fruit.