There’s a reason Kari Peter, Ph.D., associate research professor of tree fruit pathology at Penn State, speaks quickly – she often has a lot of important information to share. This was true of her multiple presentations at the most recent Great Lakes Expo, including one regarding management refinement for brown rots on stone fruits.

“If you grow stone fruit, you’ve encountered brown rot,” Peter stated. Caused by the fungus Monilinia fructicola, brown rot affects peaches, apricots, nectarines, plums and cherries. Infections begin as small brown spots, and under wet and humid conditions, ash-gray to brown tufts of fungus develop on the surfaces of the fruits.

The pathogen is spread through fungal spores. They like it warm, wet and humid. The brown rot fungus overwinters in the previous season’s fruit mummies and twig cankers. If there are too many overwintering mummies, it can even infect the blossoms. When the season starts, spores are produced on fruit mummy surfaces or on cankers. They are then dispersed by wind-driven rain. As fruit ripens, infection risk increases.

The key to preventing brown rot is to break its cycle. Peter told growers to get rid of mummies and cankers/dead wood before and during the season. Scout regularly and get rotting fruit out of trees.

Growers will also want to prevent tree stress – provide appropriate nutrients and water; use fungicides; cut out shoots during the season; and create air movement, allowing for better drying.

One of the easiest ways to reduce the spread of brown rot is to scout for and get rid of fruit mummies. Photo by Gerald Holmes, Cal Poly San Luis Obispo, Bugwood.org

Citing previous work from Dr. Norm Lalancette from Rutgers, Peter spoke about the spray strategy from dormancy to harvest to help break the brown rot cycle. In dormancy, spray copper. From bloom to petal fall, they recommend using Rovral® (FRAC 2), saving FRAC 1, 3, 7, 9 and 11 for pre-harvest use.

From petal fall to shuck split (when the pit hardens) and in first cover and second cover, try Captan® 80 WDC at 2 to 2.5 lbs./acre or Captan 4L at 1.6 to 2 quarts.

For third, fourth and fifth covers (with additional covers if they are late season cultivars), spray sulfur at 6 to 8 lbs./acre.

For the last two cover sprays before pre-harvest, they recommend Captan 80 WDG at 3.125 to 3.75 lbs./acre and Captan 4L at 2.5 to 3 quarts.

“Captan sticks around longer than sulfur, ziram and thiram,” Peter explained. “It is more effective in limiting spore germination.” She added that Captan is only needed pre-harvest if brown rot resistance is present.

However, Captan is currently under review. It has already been banned for use on certain crops by the EPA because they believe it poses “a significant health risk.” It may not be available in the future.

About 18 days before harvest, the first pre-harvest spray should rotate by FRAC group. (If there are early maturing cultivars, two pre-harvest sprays may be needed.) The second pre-harvest spray (about nine days before harvest) and the third pre-harvest spray (about a day before harvest) should also be rotated by FRAC group. If it’s a dry year, use at least one Merivon® spray (labeled for Rhizopus rot).

Peter cautioned growers to be alert to any fungicide resistance. There are large populations and rapid multiplication of the pathogen that causes brown rot. Be careful of extensive and concentrated areas of fungicide use. Also take note of the use of repetitive or sustained treatments.

She offered some final notes on what seems to be working in managing brown rot in stone fruits and what is not:

  • FRAC 3 is prone to resistance. To overcome it, increase the rate (or tank mix with another FRAC group. Tank mix with broad spectrum or another mode of action.)
  • FRAC 11 has been observed in the lab but not in the field – there’s no “practical resistance.”
  • Merivon is still performing well.
  • FRAC 1 (specifically Topsin®) also still seems to work in the Northeast.

by Courtney Llewellyn