Simple cultural practices can dramatically help reduce brown rot pressure in stone fruit orchards, but fungicides will also be necessary for control. Eradication is impossible using sanitation methods alone, says a Rutgers University pathologist.
In the eastern United States, Monilinia fructicola causes brown rot in stone fruit crops, while in the western states, M. laxa is the primary agent. In California, brown rot is caused by both M. fructicola and M. laxa, with fresh market peaches, plums, and nectarines primarily affected by M. fructicola.
The pathogen attacks all commercially grown Prunus species, including apricot, peach, nectarine, plum, prune, sweet cherry, and sour cherry, explained Dr. Norman Lalancette, extension specialist in tree fruit pathology for Rutgers University. Lalancette has worked on brown rot for many years at Rutgers’ Agricultural Research and Extension Center in Bridgeton, New Jersey. Under the right environmental conditions, complete crop loss is possible if control is inadequate, he said.
"It can very quickly knock off your crop," Lalancette said during the Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Michigan.
On a lesser degree, he noted that brown rot can cause shoot death from twig cankers, predispose the tree to other diseases, result in blossom blight, and cause postharvest fruit rot that may show up later in distribution channels. The severity of disease incidence in any given season depends on the inoculum level, fruit susceptibility, and environmental conditions.
Protecting mature fruit from the disease is difficult because wounds and natural openings in the cuticle serve as conduits for the disease, and sporulation can occur within a few days.
"When fruit is ripe, it’s very susceptible, and it doesn’t take much inoculum to cause problems," Lalancette said, adding that some stone fruit crops become susceptible during the three-week period prior to pit hardening and then again when fruit begins to ripen, lasting through harvest and to consumption.
Simple cultural controls that can lower infection and inoculum levels within an orchard include removing mummies, proper pruning, and avoiding overhead sprinklers during critical infection periods.
Mummies from the previous season are one of the primary sources of inoculum for the season, especially those that didn’t fall off the tree. The pathogen overwinters in the mummy, releasing fungus spores profusely during the blossom-blight phase and can even be a source of inoculum later in the summer for early maturing varieties, he added.
"Removing the mummies can really have an impact on managing brown rot," Lalancette said. "If you can knock them out of the tree on the ground early, during spring pruning, they can start the decay process. If you knock them out later in the fall, it takes a longer time for them to decay."
While not common, immature, green fruit, including those that were thinned and dropped to the orchard floor or those damaged by frost or insects, can also become infected and serve as an inoculum source. He noted that in dry climates like California, some orchards are flooded with irrigation water to help degrade the thinned fruit.
Lalancette said that after mummy removal, the second most important cultural control is to prune trees to improve sunlight penetration and air movement. An open canopy will help the trees dry faster after rain or dew, in addition to improving fungicide coverage.
Pruning out any twigs with brown rot cankers will also help get rid of inoculum sources.
He also advised growers to avoid overhead sprinklers during the critical fruit ripening period. "When I want to encourage brown rot infection in my research trials, I use overhead sprinklers."
Growers should determine the risk of disease incidence and choose fungicides and rates accordingly. In the eastern United States, he recommends a bloom and preharvest spray program.
Growers should spray two to three fungicide applications during bloom to reduce inoculum levels later in the season, beginning at pink stage, followed by full bloom and petal fall. Preharvest sprays can be done on a calendar basis, backing up two to three weeks from expected harvest to begin sprays, or based on fruit phenology when background color first changes.
Research trials at Rutgers show that timing sprays 18 days, 9 days, and 1 day before harvest provided greater than 95 percent control under heavy disease pressure. The initial spray is early enough to cover most of the first ripening fruit, he said, while the final spray is late enough to maintain protection during multiple fruit pickings.
According to field studies from 2001 to 2004, performed by various university researchers throughout the United States, representing 35 experiments and more than 280 treatments, he said the top four most-effective fungicides were Elite (tebuconazole), Indar (fenbuconazole), Pristine (boscalid), and Orbit (propiconazole), also available as PropiMax. Sulfur and Scala (pyrimethanil) were the least effective.
"You need to take advantage of all new chemistries to avoid resistance," he said, reminding growers to alternate and mix different fungicides to avoid using too many applications of any one chemistry, particularly in consecutive sprays.