ISSUE 5   June 11, 2009

BROWN ROT ON STONE FRUITS AND OTHER HOSTS IN NORTH DAKOTA

Stone fruits such as chokecherry, cherry, plum, apricot, and peach (all in the genus Prunus), and pome fruits such as apple and pear are susceptible to a disease known as brown rot. Brown rot is caused by three closely related fungi, but only one, Monilinia fructicola, is predominant in North Dakota. The disease rarely affects maturing apple and pear fruits, and when it does occur on these pome hosts, brown rot is reportedly associated with nearby infected stone fruits. Normally, we don’t see severe brown rot in North Dakota.

This year, an apparent outbreak of brown rot on ‘Schubert’ chokecherry (the wild type of Canada Red Cherry) is occurring in Hettinger County in a yard that is nearly completely surrounded by a hedge of chokecherry. The poor air circulation at the site (due to surrounding large trees), extensive planting of the host, probable inoculum buildup from previous years, and conducive environment (wet) culminated in a severe brown rot outbreak in 2009. Warm, wet, and humid weather favors rapid disease development. The photo below shows the extent of the damage on a hedge of chokecherry in Hettinger County. A sample of the hedge was submitted to the NDSU Plant Diagnostic Lab, and the disease was confirmed by microscopic examination of the pathogen that was found on the sample. The extensive blossom and shoot blight shown in the photo below is unusual in its extent and severity. (The brown rot text references did not prepare us for the pungent odor that was also associated with the sample!)

Brown Rot Symptoms: Early in the season, brown rot can cause blighting of blossoms, young leaves on shoots, and twigs. Injured green fruit and aborted fruit can also become infected, but healthy non-injured green fruit are usually not infected. However, such fruit will be lost if it is on infected shoots. In most years on stone and pome fruits except chokecherry, the most noticeable symptom is the rapidly progressing brown decay of maturing fruit. In North Dakota, this is most commonly seen on wild plums. Infected fruit often remains attached to the tree. On chokecherry, brown rot is most commonly seen on shoots and flowers in the spring. Under moist conditions, the fungus may sporulate, and a gray or tan-colored felt-like layer may occur on affected shoots and fruits. The fungus survives winter in the infected plant material, including the fruits which eventually shrivel up and look like fruit ‘mummies’. These mummies and other infected plant parts are an important source of primary inoculum the following spring.

Brown Rot Management: Managing brown rot relies on both cultural and chemical measures. Removing fruit ‘mummies’ and pruning out infected twigs help minimize primary inoculum the following spring. The infected material should be removed and discarded, buried, or burned. However, sanitation alone will not likely control this disease because not all infections on a plant will be found, and spores can be blown in or carried from other host plants in the area. In areas where the disease is a historical problem, and when conditions are favorable, early fungicide applications will likely be needed to reduce disease incidence.

According to the Compendium of Stone Fruit Diseases (APS Press, 1995), as few as 3-5 hours of plant surface wetness at 68°F can provide an environment favorable for infection, while 24 hours of plant surface wetness at any temperature can lead to severe infection. The fungus can infect optimally at a temperature range of 41-86°F; temperatures outside this range will slow infection but not prevent it.

For the homeowner, timing of preventative fungicide applications is typically at 4 to 5 day intervals either during periods of favorable infection or during the season of concern. For chokecherry, the season of concern is typically spring when new shoots and flowers are developing. For other fruits in which damage to fruit is the primary interest, there are typically two season of concern, one during flowering and the other beginning when fruit starts to mature. Applications during flowering are important for fruit protection because infections then can result in later fruit decay. According to the Insect and Disease Management Guide for Woody Plants in North Dakota (NDSU extension publication F1192), Captan is an effective fungicide for brown rot control. Other reportedly effective fungicides include chlorothalonil (Daconil, Bravo), myclobutanil (e.g. Spectracide Immunox), thiophanate methyl, and others. If fungicides are warranted, use only fungicides that are labeled for stone fruits, and be sure to read, understand, and follow the label.

More information can be found in the NDSU extension bulletin, Insect and Disease Management Guide for Woody Plants in North Dakota (publication F1192, revised), found at http://www.ag.ndsu.edu/pubs/plantsci/trees/f1192w.htm.  A pdf of this publication is available at http://www.ag.ndsu.edu/pubs/plantsci/trees/f1192.pdf.  

Jim Walla
Forest Pathologist
j.walla@ndsu.edu

Kasia Kinzer
NDSU Plant Diagnostician
kasia.kinzer@ndsu.edu


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