Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Conditions have been highly favorable for the development of brown rot in nectarines and peaches. Brown rot has blossom, shoot, canker, and fruit phases.
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.
Brown rot of peach.
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. Resistance has been documented to Benzimidazole (BZI) fungicides (such as Topsin M), demethylation inhibitor (DMI) fungicides such as propiconazole, fenbuconazole, tebuconazole, and metconazole, and the Quinone outside inhibitors (QoIs) including azoxystrobin and pyraclostrobin
According to Rutgers University “At least three different chemistries should be utilized in the preharvest sprays since these highly effective compounds are at-risk for resistance development. Some example programs are:
Merivon / Indar / Merivon
Luna Sensation / Indar / Luna Sensation
Flint Extra (at maximum rate) / Indar / Fontelis
Note, other DMI fungicides, such as Orbit, PropiMax, Orius, and Rhyme could be substituted for Indar.”
Sulfur and captan are labeled for use against pre-harvest brown rot: however, these products are not effective enough when utilized alone for pre-harvest brown rot management.
Again from Rutgers: “Unlike the at-risk materials, captan is a multi-site inhibitor and therefore not susceptible to resistance development. Thus, the combination of captan and use of three different at-risk chemistries provides an excellent strategy against resistance development in the preharvest fungicides.”
The key therefore to management of brown rot with fungicides is to include captan, rotate other chemistries, use highest recommended rates where brown rot pressure is high, and use proper timings.