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Leaf
Variegation or June Yellows in Strawberries
September
1996
Although
leaf variegation or June yellows in strawberries was noted and described
as "gold-striped" and "silver-striped" as early as
1719, the cause of the disorder is still not known. Various synonyms for
the disorder are Blakemore yellows, spring yellows, chlorosis, and noninfectious
variegation. Leaf variega tion occurs commonly in, but is not limited
to, strawberry cultivars that have Blakemore, Howard 17 (Premier) or Auchincruive
Climax in their parentage.
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Suspected
Causes
No causal agent has been associated with leaf variegation or June
yellows. The disorder has not been transmitted asexually by grafting
or sap inoculation but can be transmitted sexually to seedlings
of affected parents. Its appearance is unpredictable in all breeding
lines and may not become evident in a particular clone for several
years. The current theory is that leaf variegation is due to some
entity within the strawberry host cells that acts similarly to certain
virus or mycoplasma infections. Electron microscopy has thus far
not identified a virus or mycoplasma in diseased plants.
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Click
on image for larger version
Figure 1. Leaf variegation or June yellows; healthy leaf
to right.
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Symptoms
Symptoms first appear in the spring on new unfolding leaflets. A puckering
and distortion may occur in leaves with a "white streak" type
of variegation in early stages of the disease. The leaves become irregularly
mottled, streaked, or spotted with golden or pale yellow-to-white and
light green areas (Figure 1). A slight loss of plant vigor accompanies
the color change. Variegation occurs mainly during cool weather, when
the temperatures are below 50°F (10°C), in the spring or fall,
although some cultivars may remain mottled throughout the summer. Variegation
has been observed only on the foliage and flowers – never on fruit.
All runners and daughter plants produced by diseased plants are also variegated
with no reversion to a normal green color. Seemingly healthy green plants
may become variegated at any stage. There is no way to predict when a
plant or its offspring will become variegated.
As the disease
progresses, the symptoms increase in intensity. The leaves become progressively
more mottled each year until they are completely golden yellow-to-white
and frequently puckered or otherwise distorted. White-streaked or fully
yellowed leaves never regain a normal green color during the summer. Affected
plants become dwarfed. Fruit production is reduced considerably, with
the fruit being small and of poor quality. Affected plants never recover
and usually die within two to three years.
Permanent
white streaks or sectors, with or without accompanying mottling symptoms,
may occur in some cultivars affected by leaf variegation or June yellows.
White streak may be a chimera (the result of a somatic mutation). It occurs
more commonly in some cultivars, for example, in Earliglow, than in others.
Economic
Importance
The economic losses from leaf variegation are of two types. The first
and most obvious is a reduction in productivity. The degree of reduction
in fruit yield depends on the severity of the symptoms and the presence
of other diseases. Losses range from practically no reduction with mild
symptoms to a complete crop loss when the symptoms are severe. Thus, new
plantings with relatively mild variegation may be highly productive the
first year. As symptoms increase in severity, however, the yields will
decline.
The second
type of loss is the extinction of a potentially superior variety. The
cultivars Dixieland and Vermilion were formerly grown on a large acreage
in Illinois and were widely recommended for commer cial production. These
two cultivars are now nearly extinct due to variegation. The only cultivar
still on the market that has variegation is the everbearer Ozark Beauty.
This cultivar has been largely replaced by newer and improved cultivars
such as Tristar and Tribute.
Control
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Do not propagate from variegated mother plants.
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The only control to save a variety from extinction is to locate a
source of "yellows-resistant" or normal plants of the desired
variety. These plants, if found, should be propagated by virus-free
procedures to increase the number of healthy plants. Never
accept plants that show leaf variegation.
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The disorder appears to be noninfectious in that normal green clones
may grow beside variegated clones for long periods without becoming
variegated.
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Where the percentage of variegated plants in new or established plantings
is reasonably low, variegated plants should be rogued (removed and
destroyed) as soon as the disease is detected and replaced with nonvariegated,
certified, virus-free plants.
- At present there is no cure for leaf variegation, and there is no
assurance that healthy green plants in a variegation-susceptible cultivar
will remain nonvariegated
Author:
Stephen
M. Ries (s-ries@uiuc.edu)
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