The resistant isolate was originally obtained in 2006 from a commercial New Jersey peach orchard. In that year, eleven isolates of M. fructicola were collected, five from northern and central New Jersey and six from southern New Jersey. The original purpose of this collection was to obtain different genotypes of the pathogen from separate peach growing areas of the state. However, given that DMI resistance in M. fructicola has recently been reported in Georgia, Ohio, South Carolina, and New York orchards, it was decided to assay the isolate collection for resistance, even though it represented an extremely small sample size. Surprisingly, a resistant isolate was found in a sample collected from a southern NJ orchard.
How common is this resistance in New Jersey stone fruit orchards? It is entirely possible that DMI resistance is not widespread and that the single resistant isolate was found by sheer chance. However, given that the resistant isolate was discovered in such a small sample size (one out of eleven), it is more probable that DMI resistance is fairly widespread. Loss of brown rot control has not yet been reported in New Jersey, but this may be due to several interacting factors: (1) the proportion of the resistant subpopulation may still be low relative to the sensitive subpopulation; (2) mid-late summer droughts over the last two years limited development of brown rot epidemics; and (3) the use of non-DMI fungicides during bloom and fruit ripening is delaying emergence of resistant strains.
What Should Growers Do?
As noted above, no one has yet reported a control failure in New Jersey, and resistant strains have not yet been detected in Maryland or neighboring states. To delay the emergence of resistant strains, growers should follow a rotational program that alternates non-DMI fungicides from several other classes with DMIs, especially during the critical preharvest period. A number of non-DMI fungicides, singly or in combination, offer excellent brown rot control during the blossom blight and fruit rot phases of the diseases. Consult the 2008 VA-WV-MD Tree Fruit Spray Bulletin for Commercial Growers for more information. The spray bulletin may be downloaded free of charge from: http://www.ext.vt.edu/pubs/treefruit/456-419/456-419.html. Printed copies may be ordered from Virginia Cooperative Extension at $12.00/copy, including shipping. Request Publication #456-419 from:
Virginia Cooperative Extension Distribution Center
112 Landsdowne Street
Blacksburg, VA 24061
Phone: 540/231-1325 FAX: 540/231-1323
VCES will accept a check made out to "Treasurer, Virginia Tech" or a VISA or MasterCard payment by phone (540-231-1322).
For more information, contact: Anne DeMarsay