No magic bullet exists in brown rot disease control

Growers can be setting themselves up for brown rot problems with certain orchard practices.

By Melissa Hansen

Control of brown rot in soft fruits involves a systems approach, combining both cultural and chemical means to protect susceptible varieties. No single technique will ward off the disease.

Standard spray prevention schedules have not changed significantly, requiring about four sprays in California for stone fruit varieties that are sensitive to the fungus Monilinia fructicola, said Kevin Day, farm advisor for University of California Cooperative Extension, Tulare County.

Brown rot, one of the most destructive diseases of stone fruits in the United States, causes blossom blight in the spring, which develops into a twig blight and canker, and provides inoculum for latent infection of green fruit. At the end of the season, the disease develops on fruit both at harvest and during storage and transport to market.

According to Day, recommendations given to peach and nectarine growers include spraying fungicides about four times--when five percent pink is showing at blossom, at full bloom, two to three weeks before harvest, and again, just a few days before harvest. Growers should rotate their spray materials to prevent the development of resistance.

Commonly used fungicides by California stone fruit orchardists include Rovral (iprodione), Orbit (propiconazole), Benlate (benomyl), and Ronilan (vinclozolin).

Pacific Northwest stone fruit growers are much luckier than their counterparts in California, as there isn't much of a brown rot problem in Washington, said Dana Faubion, Washington State University Cooperative Extension agent for Yakima County. Stone fruit growers in the lower Yakima Valley do not have to worry about the disease, he said, although many farmers do apply sulfur and a fungicide for leaf curl.

Faubion also said there are a few stone fruit growers in Oregon's Hood River area who apply fungicides for brown rot, particularly if there has been a problem in the past. The disease has been known to be troublesome for growers in Oregon's Willamette Valley.

Researchers have yet to find a biological fungicide effective against brown rot, although they have identified cultural practices that can increase disease incidence, and are studying yeasts as potential combatants.

The influence of excessive nitrogen, composted waste, thinning practices, and yeasts upon brown rot have been studied by Dr. Themis Michailides, associate professor of plant pathology for University of California (UC), Davis. Stationed at UC's Kearney Agricultural Center in Parlier, he has examined many approaches to reduce brown rot problems.

Although it has long been recognized that fertilization practices affect levels of plant diseases, until recently, there were few good examples reinforcing that concept.

Data from research studying the effects of nitrogen on Fantasia nectarines show that blossoms from unfertilized trees resulted in the lowest levels of disease infection. More stamens were infected on blossoms from trees receiving high nitrogen rates (250 and 325 pounds of nitrogen per acre per year) than from trees that were unfertilized, or received 100- and 175-pound rates.

Additionally, the higher nitrogen treatments also showed higher disease incidence from the research inoculations in green and mature fruit. Positive correlation was also found in overwintering mummy fruit, which had higher disease incidence from trees in the high nitrogen category.

Evaluation of the fruit cuticle weight showed that cuticle thickness was greatest in the unfertilized fruit, decreasing in thickness as the nitrogen rates increased. High nitrogen treatments resulted in denser canopies, which shade fruit to cause thinner cuticles. Postharvest storage potential and brown rot susceptibility are both affected by cuticle thickness and potential water loss.

The interdisciplinary research conducted by Michailides, and others, showed that nitrogen fertilizer rates higher than 100 pounds per acre per year on Fantasia nectarines produced no beneficial effects and increased brown rot infection.

Michailides is also studying the potential of yeasts to fight M. fructicola, although he said it is too soon to report upon results. When comparing brown rot in a trial of peach trees treated with traditional fertilizers and composts to a plot treated with composted waste from urban yards, he found that the urban compost did not increase or make worse the incidence of brown rot, and may have reduced its severity.

He is further studying what may be a connection between the "green waste" and the many yeast spores that were found on the fruit surfaces.

Growers frequently overlook the significance of thinned fruit as a source of secondary inoculum in the spread of brown rot. Results from recently completed research show that brown rot was less in nectarine orchards where thinned fruit were completely removed than on those from plots where thinned fruit were left on the floor of the orchard.

For now, until that magic bullet is discovered, growers with known brown rot problems are encouraged to consider all aspects of orchard management, from timely fungicide sprays to judicious fertilization to careful orchard sanitation.