Apples that will be kept in long-term storage should be treated with preharvest or postharvest fungicides, but before growers decide which fungicides to apply in the orchard, they need to know what products will be used at the packing house. Similarly, packers need to know which fungicides have been used before harvest, says Dr. Chang-Lin Xiao, plant pathologist with Washington State University.
“Better communication between growers and packers is essential,” Xiao said during an apple quality meeting presented by WSU Extension this summer.
Infections of some diseases occur before harvest, even though the symptoms might not become apparent until after harvest. Such diseases include bull’s-eye rot, sphaeropsis rot, moldy core, and phacidiopycnis rot. Other diseases, such as gray mold and blue mold, develop after harvest when fruit is infected through wounds.
Advantages of preharvest fungicides are that growers can use products with different modes of action to manage resistance, and they can avoid the regulatory restrictions that apply to postharvest fungicides. Drawbacks are that spray coverage might not be adequate, particularly in orchards with big trees.
Preharvest fungicides include Ziram, a dithiocarbamate fungicide; the benzimidazole Topsin M (thiophanate methyl); and the strobilurin/carboxamide Pristine (which contains the active ingredients boscalid and F500). The anilide fungicide Elevate (fenhexamid) is approved for pears but is awaiting registration for use on apples.
Fungicides applied within 14 days of harvest should have a residual effect, reducing the spore loads on the surface of the fruit and protecting wounds from infection at harvest.
“Coverage is the key to the success of a preharvest fungicide program,” Xiao stressed.
He recommends making ground applications using high gallonages for best results.
Fruit should be harvested at the recommended maturity, as overmature fruit is more susceptible to decay.
Postharvest fungicides used as a drench provide thorough coverage, but a disadvantage of drench is that it might contribute to development of resistance by postharvest pathogens.
Postharvest fungicides that are available include Captan and Mertect (thiobendazole, also known as TBZ), and the newer products Scholar (fludioxonil) and Penbotec (pyrimethanil). In the past, TBZ was commonly the only fungicide used in the packing house, so benzimidazole fungicides were not recommended for use in the orchard. For example, if TBZ will be used postharvest, growers should not use Topsin in the orchard.
Scholar and Penbotec provide packers with more options for decay control and resistance management. They are effective against both blue and gray mold, Xiao said, whereas TBZ is effective against gray mold, but not blue mold. All three appear to be effective against sphaeropsis and phacidiopycnis rots, but none is effective against mucor.
Both Scholar and Penbotec have a single-site mode-of-action, which means the risk of resistance is high. Vanguard and Scala are from the same class of chemicals as Penbotec, so if either of those are used in the orchard, it can increase resistance of the decay fungus to Penbotec.
Xiao reiterated the need for better communication between growers and packers, both for decay control and resistance management. He suggested that packers keep their field horticulturists informed about their decay control program and the fungicides they are going to use.
For long-term storage, a fungicide drench is recommended if the fruit comes from orchards with a history of severe losses from gray mold or sphaeropsis. He suggested packers alternate postharvest fungicides for drenching from year to year. They should consider TBA plus Captan for drenching, but should be aware of the risk of phytotoxicity and the need to check overseas tolerances for export fruit. Drench water should be changed more than once a week, he added