PEACH DISEASE UPDATE
(Dave Rosenberger, dar22@cornell.edu
Plant Pathology, Highland)
X-Disease in Peaches
X-disease of peach trees was severe in some Hudson Valley peach orchards
last year, so it is no surprise that some trees are again developing X-disease
symptoms. Peach X-disease often follows a 10-15- year cycle. The disease
will be very severe for 4-6 years, then will gradually disappear as a
commercial problem until the next cycle begins. The reason for this cyclical
pattern has never been determined. Some X-disease was noted in the Hudson
Valley in 1997 and significant outbreaks occurred in 1998. We can expect
the incidence of X-disease to continue increasing for several more years
before it begins to subside again.
X-disease causes leaves on infected peach trees to turn yellow, curl
upward, and develop red, water-soaked spots that are not limited by leaf
veins. The leaf disorder results in early defoliation of the oldest leaves,
leaving a "horse-tail" of young foliage at the end of affected terminal
shoots.
X-disease eventually causes death of the infected scaffold limbs or of
the entire tree. Nitrogen deficiency and spray injury can also cause red
spotting on leaves, but the symptoms on trees affected by nitrogen deficiency
or spray injury usually occur uniformly throughout individual trees and
sometimes throughout entire orchards. Symptoms of X-disease affect only
random trees and/or branches of trees, although the incidence of X-disease
may be higher in locations close to inoculum sources than in more distant
parts of the orchard.
X-disease is caused by a phloem-limited mycoplasma – a minute
pathogenic organism smaller than most bacteria. The X-disease mycoplasma
is transmitted by at least eight species of leafhoppers found in New York.
White apple leafhopper, rose leafhopper, and potato leafhoppers are not
vectors. In fact, none of the X-disease vectors are abundant enough to
cause direct feeding damage and they usually escape notice in the orchard.
However, they are very efficient vectors of X-disease. The leafhoppers
acquire the X-disease organism while feeding on diseased chokecherry bushes,
on infected sweet cherry trees, or on wild seedlings of sweet cherry.
They do not acquire the X-disease mycoplasma from diseased peaches because
the population of the disease organism within diseased peach trees is
so low that leafhoppers do not encounter the organisms while feeding.
After leafhopper vectors feed on an infected plant, the X-disease
organism must grow within the insect for at least 20 days before the insect
can transmit X-disease to another plant. Once that 25-day incubation period
is completed, however, the leafhoppers with X-disease remain infective
for the rest of their lives. In laboratory studies, leafhoppers have often
lived for 30-40 days after they become infective. A single infective insect
therefore has the potential to infect numerous plants.
Peach trees that become infected with X-disease are usually inoculated
by leafhoppers during July and August, and symptoms then develop on the
trees the following year. Leafhoppers can continue to transmit X-disease
to peach trees during September and early October, but many of the late-season
transmissions fail to cause disease because the pathogen does not become
established in the plant following late-season transmissions. Relatively
mild winters during the last two years may have allowed more of the late-season
transmissions to persist through winter and may therefore have contributed
to the increasing severity of X-disease.
The most effective protection against X-disease is to isolate peach
plantings from all sweet cherry blocks and to regularly eradicate all
chokecherries within 500 ft of peach orchards. The chokecherry species
that harbors X-disease is Prunus virginiana, a plant that is more
like a shrub than like a tree. P. virginiana rarely reaches more
than 15 ft in height in eastern New York. It should not be confused with
the wild black cherry (Prunus serotina), which can develop into
an 80-ft tall tree. Prunus serotina does not develop or harbor
X-disease.
The two wild cherry species can be distinguished by the appearance
of the bark, leaves, and fruit. Leaves of P. virginiana are more
broadly oval with a hairy mid-rib on the underside of the leaf, whereas
leaves of P. serotina are more narrow and have a smooth midrib
on the underside of the leaf. Chokecherry plants infected with X-disease
develop a red-yellow fall coloration during early July. With our early
season this year, the distinctive color of infected chokecherries began
appearing about two weeks ago along roadways in the Hudson Valley.
X-disease symptoms in sweet cherry trees are often indistinct, making
it difficult to determine when a sweet cherry tree is infected. Infected
trees usually produce small fruit that ripens later than healthy fruit,
but this symptom is often indistinct in cherry orchards where fruit maturity
is already variable because of uneven crop load. Unlike peach trees, cherry
trees with X-disease can remain alive for many years after they become
infected. Such infected trees become a long-term source of inoculum for
other cherry trees and for peach trees. Whenever possible, peach plantings
in the Hudson Valley should be kept at least 500 feet away from sweet
cherry plantings.
There is no chemical means (sprays) for protecting trees from X-disease.
Leafhopper control in peach orchards may reduce the spread of disease.
Early maturing varieties of peaches may benefit from a postharvest insecticide
spray to minimize leafhopper populations in these trees during August.
However, spraying for leafhoppers is not a substitute for identifying
and eradicating inoculum sources, because infective leafhoppers can enter
the orchard from hedgerows and may infect trees before they are killed
by insecticide residues.
Injections of terramycin can be used to treat diseased trees, but
the treatment procedure is labor-intensive, must be done during September,
and must be repeated annually to prevent a relapse of treated trees. Most
growers consider it more cost-effective to remove X-diseased trees and
replant new trees in their place.
It is not necessary to remove X-diseased peach trees to prevent spread
of the disease, because peach trees do not act as a source of inoculum
for the leafhopper vectors. In young orchards, infected trees can be removed
and replanted after the source of inoculum (hedgerow chokecherries or
seedling sweet cherries) has been identified and removed. X-disease does
not remain active in the soil.
Peach Mildews
Mildew infection on peach leaves is not very common, but several
species of mildew can cause surface lesions on peach and nectarine fruit.
Infections on fruit often appear as discolored spots or rings on immature
fruit and as scabby areas on the surface of mature fruit. White spots
or rings on immature fruit can be caused by the Sphaerotheca pannosa,
the common mildew species found on peaches, nectarines, apricots, and
roses. Rusty brown spots or rings are symptoms of the disease know as
"rusty spot" and are usually caused by Podosphaera leudotricha,
the apple mildew fungus.
 Peaches
vary significantly in their susceptibility to both species of mildew.
Roses can supply inoculum of S. panosa if mildew-susceptible peach
varieties are located close to infected roses. Rusty spot can be a significant
problem if susceptible peach varieties are planted adjacent to mildew-susceptible
apples that will supply inoculum for infecting the peaches.
Peaches and nectarines are most susceptible to mildew infection between
the shuck splitand pit-hardening stages. Sulfur sprays applied during
that interval usually provide adequate protection. Under New York conditions,
there is little benefit to applying mildewcides after pit hardening. The
symptoms evident on fruit at the pit-hardening stage cannot be eradicated,
and continued spread of the disease after pit hardening is unlikely.
Next in this issue: 6/21 Insects
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