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Introduction
Downy leaf spot of hickory, also known as white mold or white leaf
spot, is caused by the fungus, Microstroma juglandis. The fungus
is capable of infecting several species of Walnut (Juglans
spp.) and Hickory (Carya spp.). Host plants include butternut,
bitternut hickory, mockernut hickory, pignut hickory, sand hickory,
shagbark hickory, water hickory, pecan, Arizona walnut, black walnut,
California black walnut, Hinds walnut, little walnut, and Persian
walnut. Downy leaf spot occurs in all regions of the world where walnuts
and hickories are grown.
Symptoms
In the spring, as leaves expand to full size, infected
trees will begin to show a light green, spotty discoloration on the
surface of the leaves (Fig. 1). These
spots can vary is size and may coalesce to form large angular lesions.
The undersurface of the leaves contain powdery, white, fuzzy spots
(Fig. 2.) that may be more concentrated
near the leaf veins.
Figure 1: Leaf surface symptoms of Downy leaf spot on Hickory.
(provided by Dawn Dailey O'Brien, Cornell
University) |
Figure 2:Underneath the leaf symptoms of Downy leaf spot on
Hickory. (provided by Dawn Dailey O'Brien,
Cornell University) |
Another symptom may be broom formation near the ends of the branches.
The leaves found in these brooms are often smaller than others and
have a yellowish appearance. The broom leaves often shrivel, wilt
and drop beginning in early summer. Brooms can become quite large
reaching up to 2 meters in diameter.
Disease Cycle
The disease cycle of Microstroma juglandis is not completely
known. Some researchers believe that the fungus overwinters as stromata
in fallen leaves. Infectious spores are released from the remains
of the fallen leaves in the spring to cause new infections.
Management
Strategies
Fungicide applications are not recommended as the disease is considered
economically insignificant. If a pleasing appearance of the trees
is desired, pruning out the brooms from within the tree's canopy may
help improve the appearance.
Last updated, KLS, 08/02
This publication may contain 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
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