Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Conditions have been highly favorable for the development of brown rot in nectarines and peaches. Many nectarine varieties are very susceptible and brown rot has been heavier this year in nectarines. Brown rot can also be a major problem in peaches. Peach varieties are somewhat less prone to brown rot that nectarines but this is also variety dependent. I have seen heavier brown rot in white fleshed varieties with smooth skins this year.
Brown rot has blossom, shoot, canker, and fruit phases. A good description of the disease can be found at the USDA Kearneysville, WV website http://www.caf.wvu.edu/kearneysville/disease_descriptions/ombrownr.html
Brown rot is caused by the fungus Monilinia fructicola. The disease overwinters in orchards with mummified fruit, infected twigs, and branch cankers. It is the fruit mummies that are the major initial source of inoculum. Good orchard sanitation to remove mummies is necessary to reduce brown rot pressure.
Blossom and twig infections that occur earlier in the year are the major source of inoculum for fruit infections. Fruit become more susceptible to brown rot as they color up and fruit with damage (branch abrasions, insect damage, bird damage, etc.) are most susceptible.
Brown rot infections need free moisture from dew or rain and infections are greatest in warm conditions (optimum temperature for infection is 77°F) with humidity above 95%. Under optimal conditions of moisture and temperature, infection takes one to six hours.
Fungicides are critical for brown rot control in nectarine and peach orchards. A good fungicide program requires two to three applications during bloom and two to three applications prior to harvest. Ripe fruit ready to be picked are very susceptible to brown rot infection. An application of a brown rot fungicide immediately before harvest (1 to 3 days) may be needed to provide fruit with an adequate shelf life. Always check the fungicide labels for days to harvest for any product used in the orchard.
In recent years, the brown rot organism has developed resistance to certain fungicides in some orchards in the east. According to Clemson University, “Benzimidazole (BZI) fungicides (such as Topsin M) were used extensively when they were first introduced in the 1970s. However, brown rot resistance developed within a few years of their introduction to the market and despite infrequent use of BZIs in the Southeast, resistant populations have persisted in South Carolina and Georgia.”
“Demethylation inhibitor (DMI) fungicides such as propiconazole, fenbuconazole, tebuconazole, and metconazole are very effective against brown rot and have been used for a long time in the Southeast. Resistance was first documented in Georgia in 2004 and has since been reported in South Carolina and other states. The good news is that increasing the rate of DMI fungicides (which is allowed for some but not all DMIs) still controls resistant populations.”
Research at Penn State has shown significant resistance to DMI fungicides in some counties but not in others.
“Anilinopyrimidines (such as Vangard or a part of Inspire Super) were not sufficiently effective in experimental trials for preharvest brown rot sprays”. Inspire Super is recommended for the region where brown rot pressure is moderate to low.
“Quinone outside inhibitors (QoIs) including azoxystrobin and pyraclostrobin (which is available only in combination with boscalid as Pristine or fluxapyroxad as Merivon) have been available since about 2000 (azoxystrobin) for brown rot control. They are single-site inhibitors as well and thus prone to resistance development.”
In work at Penn State, again resistance to brown rot with the Qol fungidides was found at high levels in some counties but not in others.
Lastly, Clemson researchers state that “Fungicides having multiple sites of action, which are not prone to development of insensitivity, are not available. While sulfur and captan are labeled for use against pre-harvest brown rot, these products are not effective enough when utilized alone for pre-harvest brown rot management.”
The key therefore to management of brown rot with fungicides is to rotate chemistries, use highest recommended rates where brown rot pressure is high, and use proper timings.
If fungicide resistance is suspected you may be interested in a program at Clemson University. They have developed a tool for profiling what fungicides are and are not working well for a specific orchard. See this brochure for more information http://www.clemson.edu/extension/horticulture/fruit_vegetable/peach/diseases/pdf/profile_flyer.pdf