I. Introduction: Phytophthora root, crown, and collar rots
are common and destructive diseases of fruit trees throughout the world. In eastern North
America, apple, cherry, peach, and apricot trees are most commonly attacked, whereas pear
and plum trees appear to be relatively resistant. True collar rot (infection of only the
scion portion of the trunk near the graft union) is relatively rare in this region,
although the term often is applied to diseased root and/or crown portions of the
rootstock, which are much more common. Trees declining from Phytophthora root and crown
rots are frequently diagnosed as suffering from"wet feet" (mistakenly assumed to
be root asphyxiation or "drowning"), and sometimes are confused with those
suffering from winter injury.
II. Symptoms: Diseased trees are most likely
to be found in heavy, wet soils or sections of the orchard where water collects or is slow
to drain (Fig. 1; healthy tree in foreground). Above-ground symptoms are variable among
tree species and locations, but include poor growth with sparse off-color foliage (Fig.
2), wilt, and collapse (Fig. 3). Infected trees may decline over more than one season, and
gradually-declining apple trees often show abnormal or premature coloration of the foliage
in early autumn (Fig. 4). In other cases, previously-healthy trees may suddenly collapse
and die shortly after resuming growth in the spring, especially if the previous autumn was
excessively wet. Similarly, previously-healthy trees may suddenly collapse during the
latter part of the growing season (Fig. 5) following a very wet spring.
A diagnostic reddish brown discoloration of the inner
bark can be seen by removing several inches of soil around the base of declining trees and
cutting away the outer bark layer on the exposed crowns; often, it's possible to see a
sharp contrast between the infected and healthy (white) crown tissues (Figs. 5 and 6; note
that the knife is inserted at the level of the soil line in Fig. 6). The inner bark of
larger infected roots often shows a similar discoloration. These symptoms distinguish
Phytophthora root and crown rots from other common causes of decline and collapse. For
instance, apple trees killed due to rootstock infections by fire blight often show many of
the same general symptoms (premature autumn coloration, general collapse), but infected
rootstocks are most often uniformly dead to the graft union, i.e., there is no contrast
between healthy and infected tissues in the rootstock portion of the tree. Winter-injured
bark on tender stone fruit trees is usually confined to the above-ground portion of the
trunk, particularly on the southwest side of the tree; in contrast, Phytophthora root and
crown rots generally are confined to below-ground tissues, although lesions may
occasionally extend up into the scion. Other causes of girdling (borers, graft union
incompatibility, certain virus infections) may also cause above-ground symptoms of decline
similar to those of root and crown rots.
Cycle and Causal Organisms: Phytophthora root and crown rots are caused by a group
of related soil-borne fungi in the genus Phytophthora. Some of these fungi are
common inhabitants of agricultural soils, whereas others are introduced on contaminated
planting stock or through the movement of contaminated soil and water. Although the
individual Phytophthora species vary somewhat in certain biological characteristics
and their abilities to attack specific tree species and rootstocks, all have one critical
trait in common: they are capable of causing significant damage only when soils are
extremely wet or saturated.
The Phytophthora fungi persist in the soil mainly as
dormant resting spores (oospores, chlamydospores) or in a vegetative growing form within
infected plant tissue. When the soil is moist or wet, reproductive structures (sporangia;
Fig. 7) are produced, either as the result of germinating resting spores or as direct
outgrowths of the active fungus within infected roots and crowns. These sporangia are
filled with the infective spores of the fungus (zoospores), which are expelled into the
soil in significant numbers only when it is completely saturated with water--that is, when
water is standing or puddled on the soil surface. The microscopic zoospores then use
tail-like structures to swim short distances through the water-filled soil pores and find
susceptible plant tissues, to which they are chemically attracted. Zoospores may also swim
to the soil surface, where they can be carried relatively long distances by runoff water
and contaminate new soils or ponds and canals used for irrigation water. The
fungus' requirement for standing water cannot be overemphasized. For instance, in a
mound or berm of soil rising above a flooded orchard floor, zoospore activity is reduced
by 90% at a level of 4 inches above the flooded zone, and there is virtually no
activity at a level of 10 inches or higher above the flooded zone.
Whether or not infection occurs once zoospores reach root or crown tissues
depends largely upon the inherent susceptibility of the rootstock and the physiological
condition it is in. Although many of the physiological factors that influence disease
development are unknown, it appears that a tree's ability to resist infection is reduced
while saturated soil conditions deprive its roots of oxygen. Therefore, episodes of soil
saturation serve as infection periods for Phytophthora root and crown rots, since they (i)
provide the conditions necessary for zoospore activity; and (ii) increase the tree's
susceptibility to disease during that time. The length of the saturated period necessary
to produce an infection can can be highly variable, depending upon a wide variety of
genetic, physiological, and environmental factors; however, the severity of an infection
period is roughly proportional to the number of days that the soil remains saturated and
how quickly it drains thereafter. The number of long saturation periods that a tree is
exposed to also is important, since additional infective zoospores are produced and
released by the fungus from each new infection site every time that conditions are
favorable. The optimum temperature for disease development varies according to the
individual Phytophthora species involved. Soil temperatures in the range of 15 to
25°C (59 to 77°F) are ideal for most, although cooler temperatures can be conducive as
Some rootstocks appear to be most susceptible to infection
during the spring and autumn, which are also the periods of the year during which soil
temperatures are most favorable for zoospore production and activity. Rootstock
susceptibility and fungus activity are both low while trees are dormant.
IV. Monitoring: Symptoms often are most diagnostic in the prebloom
period, when girdled trees either bud out and collapse or fail to make normal, healthy
growth. This is a good time to detect the reddish-brown lesions on the major roots and
crowns (Figs. 5 and 6) typical of Phytophthora infections, before the tree dies and
they begin to decay; once the tree dies, it often is difficult to distinguish between root
and crown rots or other causes of girdling. On apples, early autumn is another good time
to spot partially-girdled trees, by the premature coloring of their leaves. Typically,
young trees between 3 and 10 years of age show the highest incidence of infection, but
trees of all ages are susceptible. Trees in the early stages of decline can sometimes be
saved by initiating a fungicide treatment program; for those in more advanced stages, it
usually is cheaper to remove the tree and replant it rather than waste time and money to
postpone the inevitable.
V. Management: Management of Phytophthora
root and crown rots is best accomplished using an integrated program of cultural practices
and, sometimes, chemical control. Soils that are excessively slow to drain or subject to
periodic waterlogging should be avoided. Marginal sites should be modified as necessary to
provide the additional drainage recommended for growing tree fruit crops, e.g., install
drain tiles, create diversion ditches, rip underlying pan layers, etc. Planting
trees on ridges or berms will raise much of the crown area above the zone of significant
zoospore activity and provides an important margin of safety, especially in a wet year.
Where irrigation is practiced, avoid excessive rates of application and arrange drip
emitters to avoid puddling around the trunk.
Tree species and rootstocks should be selected to
match the soil and drainage characteristics of an orchard. However, rankings of resistance
and susceptibility to Phytophthora root and crown rots must be recognized as
generalizations only, due to the array of Phytophthora species that potentially can
be involved. Pears are the fruit tree with the greatest resistance to these diseases, and
are the most likely to remain healthy in a relatively wet site. Apple rootstocks vary
widely in susceptibility, but generally are more susceptible than pears and more resistant
than stone fruit rootstocks other than plums. Among apple rootstocks, seedling is
relatively resistant, as are M.9, M.2, and M.4; M.7 (and M.7a), M. 26, and MM. 111 are
moderately susceptible; MM.106 and MM.104 are highly susceptible. The new CG (Cornell
Geneva) rootstock series has been bred to show resistance to some Phytophthora species under greenhouse conditions, but there is insufficient experience to determine
whether this will hold under field conditions (note that MM.106 also appeared resistant in
initial greenhouse trials). 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 the heavier cherry soils. Some of the newer clonal cherry rootstocks may
have an additional measure of resistance, but these have not been sufficiently evaluated
in the field to determine. Seedling peach and apricot rootstocks are very susceptible,
although the range of suitable soils may be expanded if these fruits can be grown without
other problems on plum-type rootstocks.
Preplant soil fumigation should not be relied upon to control
Phytophthora root and crown rots, since the fumigant never completely eradicates existing
inoculum from the soil and Phytophthora spp. are easily reintroduced. Some
fungicides have activity against Phytophthora and a narrow range of related
organisms, and can be effective when used preventively in combination with the cultural
practices described above. Because Phytophthora disease occur sporadically,
it is seldom economical to treat entire orchards on a regular preventative basis.
Rather, it is best to target such applications to sections of the orchard and seasons that
are most conducive to disease development (e.g., wet spots or orchard sections with
previous Phytophthora problems, excessively wet spring and/or fall seasons). Also
note that these fungicides seldom are effective in reviving trees once the crown has
become infected and moderate symptoms of decline have appeared. Check current labels and
recommendations for approved materials and timings.
Anonymous. 1997. Introduction
to the Oomycota (Water Molds). University of California Museum of Paleontology.
Ellis, M. A. 1997. Phytophthora Root and Crown Rot of Fruit Trees. The Ohio
State University Extension Factsheet HYG-3029-95.
L., and Gubler, W. D. 1995.
Phytophthora crown and root rot. UC Pest Management
Guidelines, University of California Statewide Integrated Pest Management Project.
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Wilcox and A. R. Biggs, 1998-2005.