Reports on Plant Diseases |
| RPD No. 817 -
Necrotic Leaf Blotch of Golden Delicious Apples |
September 1984
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[ Symptoms
] [ Cause ] [ Control ]
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Figure
1. Golden Delicious leaves affected with necrotic leaf blotch (Cornell
University).
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Necrotic leaf blotch (NLB) is a common but minor disease that appears
to be restricted to the Golden Delicious cultivar of apples and its bud
sports. The disease, apparently a physiological disorder, has been observed
for about the past 20 years in most apple-growing areas of the Eastern
and Midwestern United States.
Necrotic leaf blotch usually first appears in Illinois during July and
is most severe during July and August. Mature leaves from the base to
the center of upright, succulent, rapid-growing shoots are usually the
only ones that show symptoms. Young, immature leaves on succulent shoots
and mature or cluster leaves on fruiting wood are not affected.
The effect of the disease varies among individual Golden Delicious trees
both from orchard to orchard and from tree to tree within the same orchard.
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Symptoms
Irregular brown blotches, usually 0.5 to 1.5 centimeters in diameter,
suddenly appear in the leaves (Figure 1). The blotches are usually restricted
by the larger leaf veins. Most affected leaves immediately turn yellow
and drop all at once a few days after the necrotic blotches first appear.
The most conspicuous symptom of NLB is the large number of yellow leaves
on the tree and later on the ground that suddenly appear during July and
August (Figure 2).
Necrotic leaf blotch usually appears in two to four or more distinct
waves during certain periods, scattered throughout the latter half of
the growing season. Between these periods little or no additional disease
develops. During the course of the season, 10 to 25 percent defoliation
may occur on severely affected trees in Illinois. All sports of Golden
Delicious on any rootstock appear to be equally susceptible.
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Figure 2.
Yellow leaves appearing in late summer (Cornell University).
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Cause
The causal agent of necrotic leaf blotch is unknown. The disease commonly appears
during hot, hazy weather following a humid, rainy period. Other environmental
factors or an air pollutant may be involved. The levels of ozone or sulfur dioxide
in the air and foliar nutrients in the leaves have little or no effect on disease
development. No fungal, bacterial, or other type of pathogen has been consistently
isolated or associated with NLB.
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Control
1. Golden Delicious trees sprayed during the cover period with fungicides containing
maneb (Manzate D, Dithane M-22), mancozeb (Manzate 200, Dithane M-45), zineb
(Dithane Z-78, Zineb), metiram (Polyram), or Dikar have less NLB than unsprayed
trees or trees sprayed with other fungicides.
2. Trees that have an annual moderate crop of fruit have less NLB than trees
that have a biennial bearing habit or a light fruit crop.
3. Golden Delicious trees pruned to a central leader commonly have less NLB
than trees pruned to an open center.
Note: Mention of a trade name or proprietary product does not constitute warranty
of the product and does not imply approval of this material to the exclusion
of comparable products that may be equally suitable.
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For further information concerning diseases of crucifers
and other vegetables, contact Mohammad Babadoost, Extension Specialist in Fruit
and Vegetable Pathology, Department of Crop Sciences, University of Illinois
at Urbana-Champaign.
University of Illinois Extension provides equal
opportunities in programs and employment.
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