Fruit Pest News
Volume 7, No. 21 September 7, 2006
An online newsletter whose goal is to provide all interested persons with timely information
on diseases and insects of commercial fruit and vegetable crops in Tennessee.
Text appearing in blue or red can be clicked to link to other web sites. Be aware that much of the linked
information is produced in other states and may not be applicable to Tennessee.
In This Issue:
1. Strawberry: Restriction on Canadian Plants
2. Apple: Phytophthora Crown Rot and Collar Rot
3. Apple: Cosmetic Storage Diseases
4. Apple: Prevent Storage Decays
5. Peach Variety Disease Susceptibility
1. Strawberry: Restriction on Canadian Plants
The USDA has just placed a restriction on importation of strawberry plants from Canada because of the finding of the golden nematode near the strawberry nursery area. The golden nematode does not affect strawberry - only potato - but is a regulated pest. The USDA is taking this step to protect the U.S. potato industry.
This action does not affect any Tennessee strawberry growers, because they have already purchased and received their tips, or their tips are being rooted elsewhere in the United States. By next season, Canadian officials will have had time to intensively survey the strawberry nursery fields and satisfy the USDA that they are free of the golden nematode (if they are free), thereby clearing shipments into the U.S. The areas that this restriction will affect this year are the coastal areas, particularly Florida, where planting takes place later than here. Many of those growers are yet to receive their plants. (SB)
2. Phytophthora Crown Rot and Collar
Rot of Apple
Crown rot, collar rot, and root rot in apple trees are caused
by Phytophthora species (root rots can also be caused by other types of fungi). These
three diseases can occur on one tree and can be caused by a single infection, but
are given different names to designate the part of the tree affected. Collar rot
occurs in the scion part of the tree. This disease has decreased due to changing
variety preferences and to the practice of raising graft unions above the soil line
at planting. Crown rot is a disease of the rootstock portion of the tree. It has
become more important because of the increased use of susceptible rootstocks such
as MM106 and M26. Phytophthora root rot is a disease of the root system away from
the crown region. It may occur along with crown rot or may occur by itself.
In
the spring, symptoms of Phytophthora include delayed bud break, leaf discoloration,
and poor growth. Foliage is sparse, yellowish, and may develop an early purple discoloration
in autumn. In dry weather, infected trees are the first to exhibit symptoms of water
deficit.
The most obvious symptom is a partial or complete girdling of the
trunk with reddish-brown, water-soaked areas of dead tissue at the base of roots
where they attach to the trunk. The entire underground portion of the trunk is usually
water-soaked and
brown with the dead area extending upward to the graft union.
The fire
blight bacterium can cause rootstock cankers on M26 and M9 that are almost indistinguishable
from crown rot. Fire blight cankers form in July and August following early season
blossom and/or shoot infections. Rapid decline of trees on these rootstocks in late
summer may be caused by fire blight cankers below the graft union.
Avoid planting
fruit trees in poorly drained locations, and do not allow depressions to form around
the base of trees. Where drainage is questionable, use the less susceptible rootstocks
such as seedling and M9. Moderately susceptible rootstocks are M7, M26, and MM111.
Ridomil
and Aliette fungicides are helpful in control, but should not be solely relied upon.
They should be used in conjunction with cultural practices. Because Phytophthora
diseases occur sporadically, it is usually not economical to treat entire orchards
on a regular, preventative basis. Rather, use them in problem areas. Also note that
these fungicides seldom are effective in reviving trees once the crown has become
infected and moderate symptoms of decline have appeared. Ridomil can be used in the
fall and spring only, applied as a soil spray. Dipping the roots of nursery-grown
trees into a solution of Aliette prior to planting may reduce, but not eliminate,
Phytophthora inoculum on roots. Aliette can be used as a foliar spray to nonbearing
trees. (SB)
3. Cosmetic Apple Storage Diseases
Apple
scab, sooty blotch, and flyspeck can develop after harvest,
in storage. Their appearance can reduce the marketability of the fruit. Although
these diseases first appear or increase in storage, they are the result of infections
that took place in the orchard before harvest. These diseases are favored by
wet, mild weather.
Storage scab is sometimes
referred to as 'pinpoint' scab because of its small size, from barely visible up
to 1/8 inch in diameter. The lesions are black and often shiny because of the intact
cuticle. They are circular and differ from scab lesions that develop in the orchard
by a darker color and a definite border.
Late-season scab that becomes visible
before harvest closely resembles storage scab. Late-season scab lesions often remain
viable in storage and enlarge by forming a fringe of olive-green mycelia around the
margin of the scab spot.
At optimal temperatures for the scab, sooty blotch,
and flyspeck fungi (roughly 62º to 76º F), about 8 to 10 days are required for symptom
development after infection. Symptom development does not occur below 48º F.
Control
of these cosmetic storage diseases must take place before harvest; that is, infection
must be prevented on the tree. In selecting fungicides, preharvest intervals for
the fungicides in question are a primary consideration. Then consider what diseases
to target. In orchards in which sooty blotch and flyspeck is active, Topsin
M, Flint, or Sovran are good choices. Topsin M should be tank mixed with
captan, not only for resistance management purposes, but also for bitter rot control.
Where scab has remained or has become active, Dr. Turner Sutton, North Carolina State
University, suggests using dodine (Syllit) at 1.5 to 2 lbs/acre plus captan at 5
lbs/acre. Flint or Sovran would also play a valuable role. (SB)
4. Prevent Apple Storage Decays
The
previous article discussed cosmetic apple storage diseases. Other storage diseases
such as blue mold, caused by Penicillium expansum, and gray mold, caused by
Botrytis cinerea, involve decays that actually reduce marketable yields. Unlike
the cosmetic storage diseases, most of the decay disease infections occur in storage,
mostly through wounds. Control tactics differ for the two types of storage diseases.
However, certain late-season fungicide sprays in the orchard can help reduce both
types. Applied at 14 days before harvest, a tank mix of Topsin
M with either captan or ziram helps control storage decays and sooty blotch and flyspeck.
Chemical control after harvest has become extremely limited. Sanitation practices
are more important now than ever.
If
you have had problems in the past with storage decays, you are encouraged to visit
http://www.nysaes.cornell.edu/ent/scaffolds/2002/8.19.html#d1
for details on control. The article was written by David Rosenberger, Cornell University. (SB)
5. Peach Variety Disease Susceptibility
In making your plans for ordering peach varieties for planting this winter, keep in mind their disease susceptibility. Disease resistance has not been extensively incorporated into many peach varieties. The primary disease for which we have good resistance is bacterial spot. However, that is good resistance to have, because chemical controls for b-spot are fairly limited. Refer to the following web site for a list of peach varieties and their resistance to bacterial spot: http://www.caf.wvu.edu/kearneysville/tables/bspotsus.html
(SB)
The Fruit Pest News URL is: http://web.utk.edu/~extepp/fpn/fpn.htm
Contacts:
Steve Bost, Professor and Extension Plant Pathologist
Frank Hale, Professor and Extension Entomologist
Both authors available at:
615-832-6802
fax 615-781-2568
Plant and Pest Diagnostic Center
5201 Marchant Drive
Nashville, TN 37211
Copyright 2006 The University of Tennessee. All rights reserved. This document may be reproduced and distributed for nonprofit educational purposes provided that credit is given to University of Tennessee Extension.
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