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DOGWOOD ANTHRACNOSE 



A New Disease in New Hampshire 



John Weaver & Cheryl Smith 



T. 



.he native flowering dog- 

 wood, Cornus florida, is a 

 popular ornamental tree that 

 thrives well in the warmer 

 areas of New Hampshire. 

 The southern part of the 

 state is included in the 

 northern transitional zone, 

 where patches of dogwoods 

 occur sporadically, usually on 

 south-facing slopes. One of 

 the northern most stands 

 occurs in the Barrington- 

 Rochester area. However, our 

 native and ornamental dog- 

 woods are threatened by a 

 new fungal disease. Dogwood 

 Anthracnose (also known as 

 Discula anthracnose). The 

 first cases of Dogwood An- 

 thracnose in New Hampshire 

 were confirmed recently at 

 the University of New Hamp- 

 shire's Plant Disease Diag- 

 nostic Clinic. 



In the late 1970's, unusually high 

 dieback of native flowering dog- 

 wood, Cornus florida, was observed 

 in parts of western Connecticut 

 and southern New York. The 

 disease agent Discula sp. was 

 isolated from dogwoods showing 

 similar symptoms in both the 

 eastern Cornus florida, and the 

 western Cornus nuttallii. In the 

 East, the disease spread to include 

 most of Connecticut and New 

 Jersey, southern New York, 

 eastern Pennsylvania, northern 

 Delaware and northeastern 

 Maryland, with isolated incidences 

 in Massachusetts, Virginia and 

 West Virginia. The reason for the 

 simultaneous outbreaks of the 

 disease in the East and the West 

 is uncertain. One theory holds that 

 since periods of prolonged rainfall 

 and cool wet springs are conducive 

 to the disease, recent shifts 

 favoring these climatic patterns 

 might account for the outbreaks in 

 both geographic areas. The second 

 theory is that dogwood 

 anthracnose may be a new exotic 

 disease of unknown origin. 

 Whatever, the future spread of the 

 disease is being monitored in the 



Illustration by John Weaver 



Severely diseased dogwood 



with only active branches 



in the top of the crown and 



water sprouts around the trunk. 



Trees with this level of dieback 



probably cannot be saved 



Northeast by the CAPS 

 (Cooperative Agriculture Pest 

 Survey), a informative program of 

 the USDA. 



Symptoms 



Leaf dieback first occurs in the 

 lower crown and works up the 

 tree. Leaves develop tan spots with 

 purple borders or tan blotches 

 which often expand, killing the 

 entire leaf. The white bracts of 

 flowers may also become spotted if 

 rainfall is overly abundant in the 

 spring. Infection moves from the 

 leaves to the twigs, and cankers 

 develop from leaf nodes and cause 

 twig dieback. Dead twigs appear 

 tan and are often covered with 

 black (pin-point) fruiting bodies of 

 the fungus (called conidiomata) 

 and in wet weather, the 

 conidiomata produce masses of 

 orange spores. Water sprouts 

 (succulent shoots) may develop on 

 the trunk as a result of twig 

 dieback. The disease spreads from 

 the twigs to the main branches 

 with multiple cankers coalescing to 

 girdle individual branches. If 

 unchecked, the entire tree may die 

 in ty/o to three years. 



Control 



Early detection of the disease 

 is important, because trees 

 with extensive dieback and 

 only a few active branches 

 cannot be saved and should 

 be removed. (To obtain lab 

 confirmation from the UNH 

 Plant Diagnostic Disease Clin- 

 ic on presence/absence of the 

 disease, you should submit 

 twig and leaf samples in a 

 sealed plastic bag.) If a tree 

 shows only initial signs of the 

 disease, i.e. leaf spots and 

 twig canker, pruning and 

 fungicides can save it. In the 

 spring, apply fungicide sprays 

 to protect new leaves and 

 shoots, starting at bud break 

 and repeat every 10-14 days 

 until the leaves are fully open. 

 Spray again in the fall when 

 leaves start to turn color. High 

 nitrogen fertilizers that pro- 

 duce lush succulent shoots 

 with greater susceptibility to the 

 fungus should be avoided. During 

 the summer, remove all water 

 sprouts that may develop around 

 the base. See that trees get 

 sufficient water during summer 

 droughts. However, it is important 

 to water trees at the bottoms only, 

 so that leaves and twigs remain 

 dry. Place nursery stock in well 

 ventilated areas, where they may 

 dry quickly if they should happen 

 to get wet. In the fall, rake and 

 remove fallen leaves. Also, remove 

 all dead twigs and leaves attached 

 to the tree. 



Kousa dogwood, Cornus kousa, is 

 resistant to Discula anthracnose 

 and is recommended for nursery 

 stock and replacement of dog- 

 woods that have been killed by 

 the fungus. Another possible sub- 

 stitute is Cornelian cherry, Cornus 

 mas. 



John Weaver is Survey Entomologist 

 for the Entomology Department at 

 UNH: and Cheryl Smith is a Plant 

 Pathologist at the Plant Disease 

 Diagnostic Clinic at UNH. For more 

 information call John at (603) 862- 

 1737 or Cheryl at 862-3841. ** 

 UGUST/SEPTEMBER 1991 17 



