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| Figure 57. Above-ground symptoms of Phytopthora root rot on primocanes of 'Heritage' red raspberry. |
Phytophthora Root Rot
Phytophthora root rot is caused by several related species of soilborne fungi belonging to the genus Phytophthora. To date, P. megasperma, P. cryptogea, P. citriocola, P. cactorum, and at least two additional unidentified Phytophthora species have been implicated in this disease. The disease occurs on red, black, and purple raspberries, although in the northeastern United States, it has been documented most commonly on red raspberries. The disease has not been reported on blackberries. Phytophthora root rot can be an extremely destructive disease on susceptible cultivars where conditions favor its development. Infected plants become weak and stunted and are particularly susceptible to winter injury; seriously infected plants commonly collapse and die.
Symptoms
The disease is most commonly associated with heavy soils or portions of
the planting that are the slowest to drain (lower ends of rows, dips in the
field, etc.). In fact, most declining plants that are considered to be suffering
from "wet feet" may be suffering from Phytophthora root rot. Symptoms
include a general lack of vigor and a sparse plant stand. Apparently healthy
canes may suddenly decline and collapse during the late spring or summer (Figure
57). In such cases, leaves may initially take on a yellow, red, or orange color
or may begin scorching along the edges. As the disease progresses, affected
canes wilt and die. Infected plants frequently occur in patches, which may spread
along the row if conditions remain favorable for disease development.
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| Figure 57. Above-ground symptoms of Phytopthora root rot on primocanes of 'Heritage' red raspberry. |
Because wilting and collapsing plants may be caused by other factors (winter
injury, cane borers, etc.), it is necessary to examine the root system of infected
plants to diagnose the disease. Suspect plants should be dug up, and the epidermis
(outer surface) scraped off the main roots and crown. On healthy plants, the
tissue just beneath the epidermis should be white; on plants with Phytophthora
root rot, this tissue will be a characteristic brick red (eventually turning
dark brown as the tissue decays). Sometimes a distinct line can be seen between
infected and healthy tissue (Figure 58), especially on the below-ground portion
of the crown.
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| Figure 58. Below-ground symptoms of Phytopthora root and crown rot on red raspberry. Note the sharp line of demarcation between healthy white tissue and infected reddish-brown tissue. This reddish-brown or brick-red discoloration on roots is typical of Phytophthora root rot. |
Disease Development
The fungi persist primarily as mycelium in infected roots or as dormant
resting spores in the soil. When the soil is moist, reproductive structures
(sporangia) are formed upon the infected tissue or by germinating resting spores
(oospores) in the soil. Within each of these structures a number of individual
spores called zoospores are formed. These zoospores are expelled into the soil
during periods when the soil is saturated with water. The zoospores have "tails"
(flagella) that allow them to swim through the water-filled soil pores to reach
new plant parts. Upon reaching a plant root or crown, the zoospores become attached
and begin the infective process. As water remains standing and oxygen is depleted
from the root zone, the plant is progressively less capable of resisting the
fungus's attempts at invasion, and infection becomes more likely and severe.
Each new infection site is a potential source of additional resting spores and
zoospores, allowing for epidemic disease development in sites that are subjected
to repeated periods of standing water. Although the optimum season for infection
is not known for certain, it is likely that spring and fall are particularly
favorable periods. However, it is assumed that infection can occur throughout
the growing season if soil moisture conditions are favorable.
Verticillium Wilt
Verticillium wilt is caused by the soilborne fungus Verticillium dahalie and is one of the most serious diseases of raspberries. This disease reduces raspberry yields by wilting, stunting, and eventually killing the fruiting cane or the entire plant. The disease is usually more severe in black and purple than in red raspberries. Blackberries are also susceptible to the disease, but seldom suffer severe losses.
Verticillium wilt is usually a cool-weather disease and is most severe in poorly drained soils and following cold, wet springs. The appearance of symptoms on new canes frequently coincides with water stress caused by hot, dry, midsummer weather.
Symptoms
Symptoms usually appear on black raspberries in June or early July, and
on red raspberries about a month later. The lower leaves of diseased plants
may at first appear to have a dull green cast as compared to the bright green
of normal leaves. Starting at the base of the cane and progressing upward, leaves
wilt, turn yellow, and drop. Eventually, the cane may be completely defoliated
except for a few leaves at the top (Figure 59). Black raspberry and blackberry
canes may exhibit a blue or purple streak from the soil line extending varying
heights (Figure 60). This streak is often not present or is difficult to detect
on red raspberries. In the spring following infection, many of the diseased
canes are dead. Others are poorly developed and have shriveled buds. The new
leaves are usually yellow and stunted. Infected canes may die before fruit matures,
resulting in withered, small, and tasteless berries.
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| Figure 59. Verticillium wilt symptoms on black raspberry plant. |
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| Figure 60. Bluish streaks can often be observed on black raspberry canes affected by Verticillium wilt. |
Disease Development
Verticillium is a common soilborne fungus. It causes wilt on more than 160
different kinds of plants, including strawberries, eggplant, tomatoes, potatoes,
stone fruits, and peppers. The fungus overwinters in the soil and plant debris
as dormant mycelium or black, speck-sized bodies called microsclerotia. The
fungus can survive in the soil for many years. When conditions are favorable,
microsclerotia germinate and produce threadlike fungus filaments (hyphae). These
hyphae can penetrate the root directly, but invasion is aided by breaks or wounds
in the roots. Once inside the root, the fungus grows into the water-conducting
tissue (xylem). The destruction of water-conducting tissue prevents the movement
of water from the roots to the rest of the plant. Thus, the plant eventually
wilts and dies.
Bacterial Crown Gall and Cane Gall
Crown gall is caused by the bacterium Agrobacterium tumefaciens. Cane gall is caused by a very similar bacterium, Agrobacterium rubi. Crown gall is a widespread disease of all brambles. Cane gall affects black and purple raspberries more frequently than red raspberries or blackberries. These diseases are particularly serious in nursery fields where freedom from the disease is essential. The bacteria induce galls or tumors on the roots, crowns, or canes of infected plants. Galls interfere with water and nutrient flow in the plants. Seriously infected plants may become weakened, stunted, and unproductive.
Symptoms
Young galls (tumorlike swellings) are rough, spongy, and wart-like. Galls
can be formed each season and vary in size from a pinhead to several inches
in diameter. They develop near the soil line or underground in the spring. Cane
galls occur almost exclusively on fruiting canes and usually appear in late
spring or early summer. Both crown and cane galls become hard, brown to black,
woody knots as they age (Figure 61). Some disintegrate with time and others
may remain for the life of the plant. The tops of infected plants may show no
symptoms, but plants with numerous galls may be stunted; produce dry, poorly-developed
berries; break easily and fall over; or show various deficiency symptoms due
to impaired uptake and transport of nutrients and water.
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| Figure 61. Gall on the root of a Crown Gall infected red raspberry plant. |
Disease Development
Crown gall bacteria enter the plant only through natural openings or wounds
in the epidermis or bark of the plant. The bacteria survive in infested soil
for years and can invade the roots and crowns of susceptible plants through
natural growth cracks, tissue damaged by winter injury, or damage caused by
soil insects. Man-made wounds that occur during pruning and cultivation are
important points of entry. After the bacteria enter plant tissues, an incubation
period of 11 to 28 days, or more if the host is dormant, may be required before
the bacteria induce cell proliferation, enlargement, and disorganized growth,
resulting in the production of galls. Bacteria, abundant in the outer portions
of galls, are continually sloughed off into the soil. The bacteria overwinter
in soil and in diseased galls. The following spring, these bacteria are spread
by splashing rain, water, cultivation (any practice that moves soil), pruning
tools, and insect feeding. When they contact wounded tissue of a susceptible
host, they enter and induce gall formation, completing the disease cycle.