Phytophthora root and crown rots (sometimes called collar rot) are
common and destructive diseases of fruit trees throughout the world.
In Ohio, apple, cherry, and peach trees are usually attacked. Pear
and plum trees appear to be relatively resistant. Trees declining and
dying from Phytophthora root and crown rots are frequently
misdiagnosed as suffering from "wet feet" (root asphyxiation) or are
sometimes confused with those suffering from winter injury.
Symptoms
Diseased trees are commonly found in poorly drained areas of
the orchard or yard. Heavy, wet soils that remain saturated for
extended periods of time are required for disease development.
Above-ground symptoms vary between tree species, but generally
include reduced tree vigor and growth, yellowing or chlorosis of
leaves, and eventual collapse or death of the tree. Infected trees
may decline slowly over one or more years, or they may collapse and
die rapidly after resuming growth in the spring. Trees may also
appear healthy in the spring, but die suddenly in the latter part of
the growing season. Rapid death of trees usually occurs following
excessively wet periods. On trees that decline gradually, a reddish
or purple discoloration of the leaves often occurs in autumn, while
leaves on healthy trees remain green. To observe below-ground
symptoms, you need to remove several inches of soil around the base
of the declining tree. A diagnostic reddish-brown discoloration of
the inner bark and wood can be observed after cutting away the outer
bark layer. A sharp line demarcates the reddish-brown (diseased) and
white (healthy) portion of the crown. Similar symptoms can be found
on roots, but it is generally difficult to see root symptoms without
removing the tree. This reddish discoloration and line of demarcation
between diseased and healthy tissue distinguishes Phytophthora root
and crown rot from other causes of tree decline and collapse such as
"wet feet" (drowning) or winter injury. Roots on trees killed by
excessive water are usually completely black (have no line of
demarcation) and often times have an unpleasant smell. Discoloration
from winter injury is usually confined to the above-ground part of
the trunk, particularly on the southwest side of the tree, while the
below-ground portion of the tree may still appear healthy.
Causal Organism and Disease Cycle
Phytophthora root and crown rots
are caused by several Phytophthora species. These are all soilborne
fungi, many of which are common inhabitants of most orchard soils.
Some species that are not common inhabitants may be introduced to the
orchard on contaminated planting stock or through movement of
contaminated soil. While some species are much more destructive than
others, depending on the type of fruit tree and rootstock, all
species require extremely wet or saturated soils in order to infect
and cause significant damage. These fungi overwinter and persist in
soil as mycelium in infected wood or as thick-walled spores
(oospores). Oospores remain viable in the soil for long periods of
time (years). When soils are wet, oospores germinate to form
thread-like fungal filaments (mycelia). Mycelia from germinated
oospores or from infected tissues produce reproductive structures
called a sporangia. These sporangia are filled with infective spores
called zoospores. Zoospores are released from sporangia only when
soil is completely saturated with water (standing water). The
zoospores use flagella to swim to susceptible plant tissue where they
infect. They may also swim to the soil surface and move over longer
distances in runoff water. The longer the period or periods of soil
saturation, the greater the risk of infection. Some rootstocks appear
to be most susceptible during spring and autumn, which are also the
times of year when soil temperatures are most conducive to fungus
growth and zoospore production. Rootstock susceptibility and fungus
activity are both low in the winter when trees are dormant.
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Figure 2. Disease cycle of Phytophthora root and crown rots. We wish
to thank the New York State Agricultural Experiment Station for use
of this disease cycle. It was taken from Tree Fruit Crops IPM Disease
Identification Sheet No. 7.
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Control
Control of Phytophthora crown and root rots is most
successful using an integrated program of cultural practices,
choosing the most resistant tree species or rootstock, and when
necessary, chemical control.
- Avoid sites that drain slowly or
poorly or are subject to periodic flooding. Marginal sites should be
modified (install drain tiles, create diversion ditches, rip
underlying pan layers) to provide the additional drainage recommended
for growing tree fruit crops. Planting trees on ridges or berms will
raise their crowns above the primary zone of zoospore activity and
provide an important margin of safety, especially in a wet year.
- Select rootstocks or tree species that are less susceptible to
Phytophthora and are best adapted to your individual site. Pears are
the most resistant tree fruit crop and are most likely to remain
healthy in a relatively wet site. Among apple rootstocks, seedlings
are relatively resistant. Among dwarfing-apple rootstocks, M-9, M-2,
and M-4 are relatively resistant. The Canadian rootstock Ottawa-3 has
M-9 type resistance. M-7 and MM-111 are moderately susceptible; M-26
and MM-106 are susceptible; and MM-104 is highly susceptible. Among
stone fruits, plums are relatively resistant, whereas the remainder
are susceptible to very susceptible. Mahaleb is the most susceptible
cherry rootstock, whereas Mazzard, Morello, and Colt are somewhat
more resistant and would be recommended on heavier soils
- Soil
fumigation is generally considered ineffective because it never
completely eradicates the fungus from orchard soils, and the
Phytophthora fungi are easily reintroduced into fumigated soil.
- New fungicides have recently been developed, which are effective in
controlling these diseases when used preventively, but they are
seldom effective in reviving trees once the crown has become infected
and moderate symptoms of decline have appeared. Fungicides are most
effective when used in combination with the cultural practices
described above. For the most current fungicide recommendations,
commercial growers are referred to Bulletin 506-A2, "Ohio Commercial
Tree Fruit Spray Guide." This publication can be obtained from your
county Extension agent or The Extension Publications Office; The Ohio
State University; 385 Kottman Hall; 2021 Coffey Road; Columbus, OH
43210-1044. Backyard growers are encouraged to follow the previously
mentioned cultural practices (good soil drainage) and select the more
resistant rootstocks to eliminate the need for fungicide use.
Reference
Wilcox, Wayne F. Fruit Crops IPM Disease Identification
Sheet No. 7. New York State Agricultural Experiment Station, Cornell
University Extension.
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All educational programs conducted by Ohio State University Extension
are available to clientele on a nondiscriminatory basis without regard
to race, color, creed, religion, sexual orientation, national origin,
gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and
Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868
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