Coryneum or Shothole Blight

May 2008
Kent Evans, Extension Plant Pathology Specialist (No longer at USU) • Erin Frank, USU Plant Disease Diagnostician (No longer at USU) • JayDee Gunnell, Davis County Extension Agent • Mike Pace, Box Elder County Extension Agent • Maggie Shao, Salt Lake County Extension Agent (No longer at USU)

What You Should Know

Coryneum blight, also called shothole blight, is a fungal disease of stone fruit trees including peach, nectarine, apricot, cherry, and almond (ornamental as well as nut bearing); however, the most commonly affected are apricot, peach, and nectarine. The fungal pathogen is active during cool, continuously moist, conditions and can infect and cause damage on buds, twigs, leaves, blossoms and fruit of host plants. The fungal pathogen overwinters on infected dormant leaf and blossom buds on twigs. Under sustained moisture, the pathogen can actively colonize host tissues at temperatures as low as 36 °F in as few as 24 hours, while at 77 °F, the fungus can infect a suitable host’s tissues in as few as 6 hours. Combining practices of pruning, to remove infected tissues, and applying chemical fungicides, are very effective measures used to manage this disease.

Introduction

Coryneum blight, also known as shothole blight, is a fungal disease that can cause damage on peach, nectarine, apricot, almonds (ornamental as well as nut bearing), and to a lesser degree, cherries (tart and sweet). Coryneum blight is caused by the fungal pathogen Wilsonmyces carpophilus. Changes in fungal taxonomy explain this pathogen’s name; in the past it was known as Stigmina carpophila or Coryneum beijerinckii. The pathogen can infect buds, twigs and branches, blossoms, leaves, and fruit (Figs. 1-6). The disease is most damaging in extended cool and moist conditions of spring, although this disease can occur and cause damage at anytime during lengthy wet weather.

Disease Cycle

The fungal pathogen overwinters in infected buds (Figs. 1-3) and cankers on infected twigs and branches. Spores produced from these infected tissues, in the early spring, are dispersed during rain events to infect new buds (Fig. 3). Later in the season, other susceptible tissues can become infected when there is suitable moisture on leaves and fruit. The spores (Fig. 4), called conidia, are pigmented and are extremely durable. They can survive, exposed in a dormant state, on the surface of a bud for months, waiting for just the right temperature and moisture conditions to germinate and infect its host. This pathogen often surprises growers as it is active in the early spring at cold temperatures. The fungal pathogen can infect a suitable host if moisture is continuous for 24 hours or longer at 36 °F, meaning that infections can occur when host plants are still dormant. At higher temperatures, W. carpophilus needs only 6 hours to infect a host plant under continuous moisture at 77 °F. Coryneum blight can develop rapidly under warmer temperatures, and will sporulate from infected tissues through the season during extended wet conditions.

Symptoms

Infections on leaves (Fig. 5) will develop small round purple to tan lesions that are seldom 1/4 of an inch in diameter. Infected tissues can become raised and scurfy and will often drop out as the diseased tissue cannot expand with the growing leaf. Lesions can be circular to slightly elipsoid. These diseased leaf tissues will tear along the lesion margins and may hang on at one attached point, but eventually drop out giving the shothole appearance. Infected buds will often develop a canker that can expand to girdle the twig and kill it. Often infected buds will show signs of gumming. These infected buds are easily recognized as they are often darker than healthy noninfected buds. Infection on fruit (Fig. 6) often appears first as small purple spots that become white to gray lesions, often accompanied by gumming. Infections on fruit degrades their quality and often will result in the loss of the fruit.

Management

Cultural control practices involve removing infected twigs and branches by pruning and destroying wood. Thorough pruning during the dormant season is very effective and recommended for the homeowner as a major component of management to control this disease. In the spring at shuck fall, fungicides such as Abound, Pristine, Gem, Echo 720, Bravo Weather Stik, and Ziram, are effective (see Table 1). For commercial growers, protective fungicides should be used during frequent wet weather events. During watering events, avoid wetting of branches, twigs, leaves, and fruit. In the fall, chemical controls to apply at 50% leaf drop include copper compounds such as Bordeaux mixture, copper based products like Kocide, and fungicides such as Ziram. Chemical applications in the fall will help to protect buds during the dormant season. With regard to any chemicals mentioned, read the label and follow the labeled instructions for their use. Mention or exclusion of trade names in this publication does not represent or imply endorsement nor criticism of any chemical product. See Table 1 for more details regarding chemical controls.

Diagnosis

 

 

Table 1. Chemicals recommended for the management of Coryneum or shothole blight of stone fruits may not be sold in volumes that are convenient to the homeowner; in higher volumes these can be rather inconvenient and expensive. Chemicals are listed, primarily, to inform a homeowner so they can effectively communicate with a licensed chemical applicator. Labeled chemical recommendations often change from one season to the next, always read and be familiar with labeled instructions for the use of the chemical. Specimen labels for these, and many other, compounds are available at www.cdms.net on the Internet. Another reference is the Utah Home Orchard Pest Management Guide (https://extension. usu.edu/files/publications/factsheet/home-orchard-guide08.pdf). Specific mention or exclusion of Web sites, their address, or their content represents neither endorsement nor criticism of any that are available for view on the Internet. If you have questions, contact your county Extension office.

Chemical	Active Ingredient(s)	Rate (always read the label)	Timing of application
Abound1	Azoxystrobin	11-15 fl. oz./acre/season	See label.
Pristine	Pyraclostrobin + Boscalid	14.5 oz./acre (do not exceed 5 applications per season, see label)	See label.
Gem	Trifloxystrobin	6-8 oz./acre	See label.
Bravo Weather Stik	Chlorothalonil	3.125 to 4.125 pints/acre	See label. Fall application
Ziram 76DF	Ziram (a zinc compound, see label)	See label, varies by crop.	See label. Varies by crop.
HI-YIELD Bordeaux mix fungicide2	copper sulfate + lime (a mixture of 8 lb copper sulfate + 8 lb lime/100 gal water)	See label.	See label. Fall application
Kocide 20002	copper hydroxide	See label, varies by crop.	See label. Fall application
Basic copper 532	basic cupric sulfate	See label.	See label. Fall application
C-O-C-S WDG2	basic cupric sulfate + copper oxychloride	See label.	See label. Fall application

¹Avoid spraying apples with Abound, it is extremely phytotoxic to some cultivars and can cause excessive leaf drop/defoliation.
²Pay special attention to labeled directions regarding when copper compounds can be used relative to tree physiological stage of development. Copper compounds can cause phytotoxicity, leaf scorching, and fruit russeting if used improperly. Fall application of these compounds is less likely to cause damage as trees are approaching dormancy and fruit have already been harvested.;

References

• Ogawa, J.M., Zehr, E.I., Bird, G.W., Ritchie, D.F., Uriu, K., and Uyemoto, J.K. 1995. Compendium of stone fruit diseases. American Phytopathological Society, St. Paul, MN.
• Thomson, S.V., and Ockey, S.C. 1998. Coryneum blight of stone fruits. Utah State University Extension Fact Sheet. Plant Disease Control No. 25a.