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Introduction
Hawthorn leaf blight, caused by the fungus Diplocarpon mespili,
(conidial state = Entomosporium mespili) is also known as Entomosporium
leaf spot. The conidial state of this fungus may infect and cuase
leaf spots on many species of plants, most in the rose family. The
disease first shows up as a leafspot in early summer. Some hawthornes
are so severely infected that trees may drop all their leaves by mid
or late August (Fig. 1). Diseased trees,
weakened after a few years, succumb more easily to adverse weather
conditions and other diseases. English hawthorn (Crataegus oxyacantha)
and Paul's scarlet hawthorn (C. oxyacantha 'Pauli') are more
susceptible to this disease than are other hawthorns.
Symptoms
The initial symptom is the appearance of small, angular redbrown
spots up to 14 mm in diameter. The spots form mostly on the upper
leaf surface. Tissue immediately surrounding the spots may remain
green while leaf tissue beyond that yellows. As the spots enlarge,
they coalesce and blight the leaves, and infected leaves yellow (Fig.
2) and fall prematurely. In addition, lesions may develop
on the twigs and branches. These lesions, called cankers, become elongated
and sometimes girdle and kill the shoot resulting in dieback. With
a hand lens, numerous tiny, black dots can be seen in the center of
the leaf lesions after that tissue has died. These dots are fruiting
bodies (acervuli) of the fungus.

Figure 1: Severe defoliation. |

Figure 2: Infected leaves. |
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Disease cycle
Two types of fruiting bodies are produced.
The asexual state produces conidia (in acervulia) which cause infection.
The sexual state produces ascospores in apothecia, but their role
in producing infection is unclear. Both stages overwinter in infected
leaves that drop to the ground. Conidia from dead leaves or from twig
cankers may be windblow--or rainsplashed to the newly emerging leaves
and begin the infection process again. By mid
July the fungus produces spores that cause more infections. Severely
diseased leaves fall to the ground and, after a cold winter, serve
as a source of infectious spores again the following spring.
Management Strategies
As this fungus normally overwinters in dead leaves, gather and destroy
or compost fallen leaves in autumn. Plants with dieback resulting
from chronic defoliation and branch infections should be pruned to
remove dead wood. Destroy or discard prunings. Disinfest the pruning
tools between each cut by swabbing the blades with a solution containing
7 parts rubbing alcohol and 3 parts water. If needed, homeowners in
New York State may be able to use some copper products
to treat this disease on small trees. Additional products may be available
for commercial applications. Commercial applications, please refer
to the appropriate commercial pest management guidelines, or contact
your local Cooperative Extension Office for more information on currently
registered products. Apply the initial spray as flower buds are opening
and reapply twice more at 10 day intervals or according to label directions.
Additional fungicide applications may be necessary during rainy growing
seasons. Plant resistant hawthorns such as cockspur (Crataegus
crusgalli), Washington (C. phaenopyrum),
Toba (C. mordenensis
cv. 'Toba') or Lavalle's (C. lavallei) to avoid this disease.
Created, KLS, 8/99; Updated, SLJ, 1/07
This publication contains pesticide recommendations. Changes in pesticide
regulations occur constantly, some materials mentioned may no longer
be available, and some uses may no longer be legal. All pesticides distributed,
sold, and/or applied in New York State must be registered with the New
York State Department of Environmental Conservation (DEC). Questions
concerning the legality and/or registration status for pesticide use
in New York State should be directed to the appropriate Cornell Cooperative
Extension Specialist or your regional DEC office. READ THE
LABEL BEFORE APPLYING ANY PESTICIDE. __________________________________________________________________________________
The Plant Disease Diagnostic Clinic
at Cornell University is located at 334 Plant Science Building,
Ithaca, NY, 14853. Phone: 607-255-7850, Fax: 607-255-4471,
Email: kls13@cornell.edu
or slj2@cornell.edu |