GENERAL MALAISE
(Dave Rosenberger dar22@cornell.edu,
Plant Pathology, Highland)
Rust Diseases
Cedar apple rust lesions are now apparent on leaves of susceptible
cultivars that were not protected with fungicides. Lesions appear as bright
yellow or orange dots on the upper surface of the leaves. Infections this
year are unusually severe because of the heavy inoculum load in cedar
trees. The intermittent but extended wetting periods that we had between
May 3 and May 9 provided ideal conditions for infection. Heavy rains tend
to wash spores out of the air whereas the intermittent wetting optimizes
the process of spore production, release, and dissemination.
Severe quince rust infection is also evident in our test plots at
the Hudson Valley Lab. Quince rust causes deformed fruitlets, but does
not cause any symptoms on leaves. Fruit are most susceptible to quince
rust from Tight Cluster until the time blossoms open. On unsprayed Jerseymac
trees, we noted that most of the fruitlets infected with quince rust are
dropping from the trees as our thinning treatments take effect. The selective
thinning of quince rust-infected fruit probably resulted from the fact
that most infections occurred on side flowers rather than on king flowers.
The critical infection period for quince rust was probably the split wetting
period of May 3-4 when early-flowering varieties were at King Bloom. Because
the king flowers were already open, the king fruit was less susceptible
to infection during this critical infection period. The thinning treatment
(carbaryl plus NAA at 5 ppm) effectively removed most of the side flowers
and in the process eliminated much of the quince rust. Had the infection
period occurred several days earlier, I suspect more king flowers would
have been infected and the thinner would not have removed the quince rust.
In years with heavy cedar apple rust infection, rust-resistant cultivars
often develop leaf spots that are associated with cedar apple rust infections.
Rust-induced leaf spotting can occur on cultivars such as Empire, Cortland,
and Liberty even though the trees of these cultivars will never develop
typical cedar apple rust lesions. (These cultivars occasionally have normal
lesions of hawthorn rust, but hawthorn rust is much less common than cedar
apple rust.) If rust-resistant cultivars are sprayed with a fungicide
that does not control rust (e.g., captan), then the rust spores landing
on leaves will germinate and begin the infection process before the fungus
dies. Infections may kill only a few cells, or they may appear macroscopically
as a pin-point yellow or orange spot on the upper leaf surface.
See
IPM site's information on cedar apple rust.
Rust-induced leaf spots develop after the rust fungus dies. The cells
that were killed or damaged by the rust infections are then invaded by
Botryosphaeria, Alternaria, or Phomopsis. These fungi use
the dead or dying cells in the original rust lesion as a food-base that
allows them to enlarge the original rust lesion. The resulting leaf spots
are indistinguishable from frog-eye leaf spot except that rust-induced
leaf spots are usually more uniformly distributed throughout the tree
canopy. Sometimes the original orange-yellow rust lesion is visible in
the center of the brown leaf spots initiated by rust infections.
As noted in the last issue of Scaffolds, rust diseases have no secondary
cycle in apples trees. Therefore, there is no reason to adjust current-season
spray programs if rust lesions or rust-induced leaf spotting appears in
trees. The damage for this season has been done and will not get any worse.
Fire Blight
The MaryBlyt model predicted that blossom blight symptoms should
appear in the Hudson Valley around May 23 if any infections occurred during
the high risk periods for blight infection that were registered May 6
and 7. Pears were at greatest risk during that period. The hot weather
during the past few days should cause terminal shoots to turn black and
wilt if blossoms were infected in early May.
Nectria Twig Blight
This disease, which can easily be confused with fire blight, is just
beginning to appear in Rome trees. Nectria causes terminal shoots to wilt
and die, with shoot tips forming "shepherd's crooks" similar to those
caused by fire blight. The disease occurs when the fungus Nectria cinnabarina
invades pulled or broken stems after harvest and progresses into the twigs
below the stem. After invading pulled or broken stems, the fungus continues
to grow into the fruiting node and eventually girdles the twigs. Terminal
shoots beyond the infection point wilt suddenly after the twig is girdled.
During late June and July, bright orange fruiting structures 2-3 mm in
diameter will erupt through the bark of the node just below the pulled
stem. Nectria twig blight is most common on Romes and other cultivars
that have enlarged nodes where flowers are produced.
Nectria twig blight can be differentiated from fire blight by the
fact that the Nectria cankers are usually associated with pulled stems.
Also, cankers caused by nectria twig blight rarely extend more than an
inch or two back into the tree from the affected node, whereas fire blight
cankers can "run" considerably further. When affected twigs are sliced
open, there is usually a sharp transition between healthy wood and necrotic
tissue in a Nectria twig blight canker, whereas fire blight cankers frequently
have indistinct canker margins at this time of year. Later in the summer,
the orange fruiting structures that are unique to Nectria provide a critical
diagnostic sign. Diagnosis can be complicated when both diseases are present
in the same orchard. The presence of Nectria twig blight makes scouting
for fire blight very difficult.
Although the wilted and dying shoots can make a tree look sick, Nectria
canker generally causes minimal damage to trees. No fungicides have proven
effective for limiting spread of this disease. Nectria twig blight occurs
sporadically, but infections are usually most common and abundant in low-lying
blocks with poor air-drainage (i.e., blocks subject to cold damage.)
Next in this issue: 6/1 Insects
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