Fungal Diseases 2

CAUSAL AGENTS:
Alternaria alternata f.sp. cucurbitae, Botrytis cinerea, Choanephora cucurbitarum,
Fusarium spp., Lasiodiplodia theobromae, Myrothecium roridum, Penicillium digitatum,
Phomopsis cucurbitae, Phytophthora spp., Pythium spp., Rhizoctonia solani,
Rhizopus stolonifer, Trichothecium roseum and other fungi.
DISTRIBUTION:
Worldwide
SYMPTOMS:
Symptoms vary depending on environmental conditions and the fungal fruit
pathogen(s) present.
CONDITIONS FOR DISEASE DEVELOPMENT:
Fruit-rotting fungi enter fruit when the fruit contact damp soil. Wounds and attached
blossoms are also points of entry for fruit-rotting fungi. Fruit are more likely to be
infected when relative humidity is high or if free moisture is present on fruit surfaces.
Chilling damage also makes fruit more susceptible to infection by fruit-rotting fungi.
CONTROL:
Crop management practices that prevent fruit contact with the soil surface, reduce
fruit injury especially during harvest and packing, and reduce post-harvest free moisture
on fruit surfaces will help decrease fruit rot incidence. In some cases post-harvest
application of fungicides may help reduce fruit rot losses. Post-harvest treatment of
fruit with hot water and fungicides show limited success in controlling latent infections.

 

 

FUSARIUM CROWN AND FOOT ROT OF SQUASH

CAUSAL AGENT:
Fusarium solani f. sp. cucurbitae
DISTRIBUTION:
Africa, Asia, Europe, and North America
SYMPTOMS:
The disease is most serious on squash and pumpkin, although it may also be found
on watermelon, cantaloupe and cucumber. Fusarium solani f. sp. cucurbitae race 1
attacks roots, stems and fruit, but Fusarium solani f. sp. cucurbitae race 2 attacks
only fruit. Symptoms are similar to Fusarium wilt and include stunting of the plant,
and mid-season wilting of the entire plant. However, with Fusarium crown and foot
rot there is a distinctive dark brown necrotic rot of the crown and upper portion of the
tap root. This decay extends around the stem and girdles the plant. The affected area
turns soft and mushy. During humid weather, white mycelial growth may be found on
the affected area. The roots of the plant can also become infected but the pathogen
primarily is limited to the crown and fruit of the plant. When fruit are attacked a firm
dry rot develops. The decayed areas are circular and may develop a concentric
ring pattern.
CONDITIONS FOR DISEASE DEVELOPMENT:
The fungus can survive in soil although not for long periods of time. Early infection
may result in seedling damping-off. Fusarium solani f. sp. cucurbitae can also infect
fruit which are in contact with infested soil. Severity of fruit rot is dependent on soil
moisture and inoculum density.
CONTROL:
Plant fungicide-treated seed to reduce disease initiated from infected seed. The
fungus outside of a host is not long-lived in the soil, therefore a three-to-four-year rotation out of cucurbits is usually adequate to control the disease.

FUSARIUM ROOT AND STEM ROT OF CUCUMBER

CAUSAL AGENT:
Fusarium oxysporum f. sp. radicis-cucumerinum
DISTRIBUTION:
Worldwide
SYMPTOMS:
Cucumber and muskmelon (e.g., cantaloupe, honeydew) are very susceptible to
Fusarium oxysporum f. sp. radicis-cucumerinum. In cucumber, initial symptoms
manifest six to eight weeks after sowing as pale yellow lesions at the stem base.
These lesions may expand and spread to cause a root and stem rot. As the disease
progresses, stems are colonized by the fungus leading to breakdown of cortical
tissues. In severely affected plants, pinkish-orange masses of macroconidia and
microconidia and/or a cottony-like mycelial growth may be observed on the outside
of the stem. Plants bearing high fruit loads may eventually turn brown and die,
especially when grown under high temperatures.
CONDITIONS FOR DISEASE DEVELOPMENT:
This fungus can survive for several years as thick-walled chlamydospores in soil or
embedded in plant debris. Dispersal of Fusarium oxysporum f. sp. radicis-cucumerinum
within and between fields occurs in crop debris, wind-blown soil, rain and irrigation
water, and field crews and equipment. Infection commonly occurs at the root tips or
through roots wounded during transplant and cultivation.
In protected culture, Fusarium oxysporum f. sp. radicis-cucumerinum can colonize
artificial growing media (e.g., blocks and/or slabs). Spread of this fungus occurs
through root-to-root contact or by aerial dispersal of macroconidia and microconidia.
Pruning creates wounds which are ideal entry sites for airborne macroconidia and
microconidia. Disease development is favored by cool air temperatures and soil
temperatures ranging from 17–20ºC (63–68ºF).
CONTROL:
Two key measures aid control of Fusarium oxysporum f. sp. radicis-cucumerinum:
crop rotation and a strict sanitation program for facilities, equipment, tools and
people. Early implementation of a strict sanitation program is critical to preventing
this disease in protected culture, as infection occurs most often within the first four
weeks of a crop cycle. Employ routine scouting and immediately remove all weak
and unhealthy transplants. Following harvest, dispose of all crop debris and materials
used during production, and all artificial media that contained infected plants. Infected
plant debris may be incinerated, buried or taken to a landfill. Sanitize greenhouses
and or other growing structures with a recommended disinfectant. If growing media
must be reused, it should be steam sterilized, although starting with new substrate is
preferable. Resistance to Fusarium oxysporum f. sp. radicis-cucumerinum has been
identified in rootstocks of Cucurbita spp. (e.g., C. ficifolia, C. moschata). However,
there are no known sources of resistance in cucumber (Cucumis sativus). Currently
there are no registered fungicides targeting this Fusarium species. The biological
control agent Clonostachys rosea f. sp. catenulata has been shown to suppress
Fusarium oxysporum f. sp. radicis-cucumerinum in cucumber grown hydroponically
in an artificial substrate.

FUSARIUM WILT

CAUSAL AGENTS:
Fusarium oxysporum f. sp. cucumerinum (cucumber)
Fusarium oxysporum f. sp. melonis (cantaloupe)
Fusarium oxysporum f. sp. niveum (watermelon)
DISTRIBUTION:
Worldwide
SYMPTOMS:
These fungi can infect host species at any stage of plant development.
Pre-emergence rot and damping-off can occur when soil temperatures are cool
[18–20ºC (64–68ºF)]. However, infection of older plants is most common. Wilted
leaves may exhibit chlorosis and later, interveinal necrosis. Initially one or more
runners wilt and later the entire plant wilts. In some cases, sudden collapse occurs
without any chlorosis of the foliage. Mature infected plants with high fruit loads are
prone to plant collapse. White mycelial growth may be visible on the stem at the base
of infected plants. Vascular discoloration can be seen in both the roots and stems. In
advanced stages, the roots begin to decompose, which may lead to plant death.
CONDITIONS FOR DISEASE DEVELOPMENT:
Fusarium wilt is favored by warm soil temperatures. Wilting of the lower leaves
occurs at warm temperatures [25–28ºC (77–82ºF)] and the leaves recover under
cooler temperatures. The fungus survives as chlamydospores in the soil and in plant
debris. The chlamydospores are readily disseminated in soil and in debris during
cultivation of fields, in irrigation water, by wind-blown soil and workers.
CONTROL:
For all cucurbit species, grow resistant varieties when available.
Open Field: Graft onto resistant rootstock (e.g., Cucurbita ficifolia). Soil solarization
has been shown to be beneficial in light to moderately infested fields. Maintaining a
soil pH of 6.5 and use of a NO3 nitrogen source can help reduce disease incidence
and severity. Implement an equipment and worker sanitation program to control
movement of the pathogen between fields. Crop rotation out of cucurbits for a
minimum of five years may provide control in some cases, although is not always
an effective strategy due to long-term persistence of chlamydospores in many soils.
Protected Culture: Implement a strict sanitation program for workers, tools,
equipment and structures. Produce crops in artificial substrate and do not reuse
substrate. Graft crop species onto a resistant rootstock (e.g., Cucurbita maxima,
Cucurbita ficifolia). Collect and remove pruning debris from site or destroy by burning
or burying. Remove infected plants upon recognition. Crop production during cooler
months may reduce disease incidence and severity since Fusarium wilt is not favored
at lower temperatures.

GUMMY STEM BLIGHT

CAUSAL AGENT:
Didymella bryoniae (anamorph: Phoma cucurbitacearum)
DISTRIBUTION:
Worldwide
SYMPTOMS:
Didymella bryoniae is a foliar pathogen that can infect all cucurbit species. Young
seedlings can quickly damp-off following infection. On older plants, leaf symptoms
appear as circular, dark tan to black spots surrounded by a yellow halo. Over time
these lesions dry, crack and fall out, which is often called “shot-holing.” Infection at
the leaf margin begins as a wilt and progresses toward the center, resulting in leaf
blight. Infected stems may develop cankers which produce a characteristic red or
brown, gummy exudate. Severely infected stems may be girdled, resulting in vine
death. Tiny black fruiting bodies (pycnidia or pseudothecia) may develop within the
infected leaf or stem tissue. Fruit symptoms can range from small, water-soaked,
oval or circular spots to large necrotic panels. Black fruiting bodies may develop
within the lesions. Infection can lead to softening at the blossom end which may
be brown or green. When the pedicel is infected, fruit abortion may occur.
CONDITIONS FOR DISEASE DEVELOPMENT:
Open Field: The fungus overwinters on infected cucurbit crop debris and cucurbit
volunteers. This pathogen can also be seed-borne. Wounds caused by pruning,
insects or field work can be important entry points for the fungus. The disease is
most serious in open field production during periods of moderate temperatures and
wet weather. The optimum temperature range for infection is 20–25°C (68–77°F).
Protected Culture: Cool night temperatures and high humidity favor disease
development. Infection of open flowers can lead to serious fruit quality problems,
making fruit unmarketable.
CONTROL:
Avoid overhead irrigation. Rotate out of cucurbits for a minimum of three years to
non-susceptible hosts in order to break the disease cycle. Control weeds and remove
wild cucurbits from potential field locations. Implement a preventative fungicide
spray program. Use fungicide-treated seed. In addition to the above measures, soil
sterilization and a strict sanitation program for equipment and personnel should be
implemented in protected culture.

MONOSPORASCUS ROOT ROT AND VINE DECLINE

CAUSAL AGENT:
Monosporascus cannonballus
DISTRIBUTION:
Worldwide
SYMPTOMS:
Initial symptoms include stunting and poor growth of plants. However, this may go
undetected if an entire field is uniformly affected. The older crown leaves begin to
turn chlorotic, wilt and collapse within weeks of harvest. Within five to ten days of the
first foliar symptoms, most of the canopy may be killed. Tan to reddish-brown lesions
form on the roots. Root infection leads to a loss of feeder roots. Eventually the root
system may become necrotic, resulting in plant death. Large, black perithecia form on
dead roots and are often visible. Fruit of diseased plants are smaller or cracked and
may abscise from the pedicle before ripening and have reduced sugar content. Fruit
may also become sunburned due to lack of foliage. Stem lesions are generally lacking
and above ground symptoms may be confused with other vine declines.
CONDITIONS FOR DISEASE DEVELOPMENT:
Infection by Monosporascus cannonballus is believed to occur early in the season;
however, tissue colonization is encouraged as the soil temperature increases. This
rise in the soil temperature encourages perithecia formation in the roots. Ascospores
are the long-term survival structures of the fungus. Disease spread is by movement
of infested soil or infected plant material.
CONTROL:
Management of Monosporascus cannonballus has proven to be difficult due to its
heat tolerance and thick-walled resting structures. Avoid planting melons and watermelons
in known infested fields. Also, avoid excessive irrigation, which may only delay
plants collapse. Allowing infested roots to dry out in the field followed by fumigating
soon after harvest has shown to be beneficial. The use of rootstocks has been
beneficial in watermelon, although additional work is needed for melon. Chemigation through drip irrigation lines has also been shown to be effective.

PHYTOPHTHORA CROWN AND ROOT ROT

CAUSAL AGENTS:
Phytophthora capsici
Phytophthora spp.
DISTRIBUTION:
Worldwide
SYMPTOMS:
Phytophthora capsici and other Phytophthora spp. can cause a range of symptoms
in cucurbits from damping-off in young seedlings to crown and root rot, leaf spots,
foliar blights and pre- and post-harvest fruit rots in mature plants. Symptoms of
Phytophthora crown and root rot often manifest quickly, with plant death occurring
within a few days of symptom onset. Often, affected plants exhibit a sudden,
permanent wilt. Stems can collapse while the foliage of wilted plants remains green.
Water-soaking develops in the roots, crown and lower stem near the soil line. Tissues
become soft, turning from a healthy white to tan-to dark brown. In advanced stages,
the lateral roots slough off and eventually the entire root system may be destroyed.
CONDITIONS FOR DISEASE DEVELOPMENT:
These Phytophthora spp. survive from one season to the next in infected cucurbit
tissue or in tissue of one of its other hosts (e.g., eggplant, pepper and tomato). This
organism is able to disperse itself as zoospores that are released from sporangia into
surface and irrigation water, and also as sporangia that are released into the air.
Over-irrigation, heavy rainfall and poor drainage favor Phytophthora crown and root
rot. High temperatures in mid- and late-season further stress already weakened
plants and the disease can progress rapidly. Phytophthora crown and root rot
incidence is greatest in low lying areas of fields where soil remains saturated for
extended periods. Increasing the frequency and/or duration of irrigation will increase
the incidence of this disease. Favorable conditions for this pathogen include soil
temperatures above 18°C (65°F) and prolonged wet periods with air temperatures
between 24–29°C (75–85°F).
CONTROL:
No single method is available to provide adequate control of Phytophthora crown and
root rot. Cultural practices shown to reduce infection include irrigation management
(e.g., drip irrigation) and high plant beds to improve drainage. When possible, avoid
crop rotation with pepper and, to a lesser degree, other solanum species (e.g.,
eggplant, tomato). Additional practices to manage the disease include pathogen
exclusion by following good sanitation and cultural practices. Fungicide sprays and
soil drenches have also been shown to be effective.