Diseases

BEET PSEUDO-YELLOWS
CUCUMBER MOSAIC
CUCUMBER VEIN-YELLOWING
CUCURBIT APHID-BORNE YELLOWS
CUCURBIT YELLOW STUNTING DISORDER
GEMINIVIRUSES

BEET PSEUDO-YELLOWS

CAUSAL AGENT:
Beet pseudo-yellows virus (BPYV)
VECTOR:
Greenhouse whitefly (Trialeurodes vaporariorum)
DISTRIBUTION:
Australia, France, Italy, Japan, The Netherlands, Spain and USA
SYMPTOMS:
Beet pseudo-yellows is an important disease on greenhouse cucumber and muskmelon
(cantaloupe). This virus was previously known as cucumber yellows or muskmelon
yellows. Symptoms first appear on older leaves as yellow spots which develop into
yellow blotchy raised areas between veins, while the veins themselves remain green.
These raised areas eventually coalesce to form large thickened areas, which become
brittle and may disintegrate. As the disease progresses, younger leaves begin to
develop symptoms, but fruit remain unaffected. Plants infected at an early stage
can be stunted and may have fewer fruit. Symptoms caused by beet pseudo-yellows
can easily be confused with symptoms resulting from nutritional deficiencies (e.g.,
magnesium), insect feeding, poor growing conditions and premature aging.
CONDITIONS FOR DISEASE DEVELOPMENT:
The greenhouse whitefly can acquire and transmit Beet pseudo-yellows virus in a
semi-persistent manner. Symptoms begin to develop two to four weeks after infection.
The virus is not seed-borne or mechanically transmitted. High light intensity appears
to be necessary for disease development. This virus has a large host range among
crops and weed species. In addition to cucumber, muskmelon (cantaloupe) and
squash, BPYV also infects many ornamentals and other vegetable crops such as
lettuce, endive, carrot, spinach and beet.
CONTROL:
Prevent whiteflies from entering protected culture facilities by screening openings
with insect-proof netting (minimum 50–52 mesh/297 micron screen). Implement a
comprehensive insecticide program, crop rotation and a host-free period. Eliminate
intercropping of young and old plants to reduce inoculum levels. Remove weeds and
volunteer plants in and around greenhouses. Dispose of plant debris immediately
after harvest to eliminate inoculum sources.

 

CUCUMBER MOSAIC

CAUSAL AGENT:

Cucumber mosaic virus (CMV)
VECTOR:
Several aphid species
DISTRIBUTION:
Worldwide
SYMPTOMS:
All cucurbits are susceptible to CMV, but watermelon is rarely affected. Symptoms
may vary based on host, environment and age of the plant at time of infection.
Symptoms first appear on younger leaves, which curl downward and become mottled,
distorted and reduced in size. Plants may become stunted with shortened internodes,
resulting in a rosette-like appearance of the youngest leaves. If infection occurs after
flowering, vine growth may not be reduced, but fruit may be mottled and distorted.
Cucumber: Seedlings seldom show symptoms during the first few weeks of
growth, but symptoms can appear once growth becomes vigorous. Leaves become
mottled and distorted with downward-curling edges. All subsequent growth is
reduced, leaving the plants dwarfed. Older leaves may develop chlorotic margins,
which later become necrotic. Fruit are often misshapen, mottled (yellowish-green),
warty and reduced in size. Infected fruit may appear bleached due to a lack of
chlorophyll production.
Melon: Melon plants may exhibit severely stunted growing tips. Even if fruit does not
express distinct symptoms, overall fruit quality is often poor.
Pumpkin: Early infection often results in severe foliar mosaic. Fruit may be rendered
unmarketable due to mosaic symptoms.
Squash: Early-season infection can result in severely stunted plants with deformed
leaves. Petioles often exhibit a downward or bending growth pattern. Leaves may also

be reduced in size. Fruit can become unmarketable due to pronounced rugosity
of the fruit surface. In summer squash, warty raised yellow areas appear on the fruit,
and are surrounded by dark green areas.
Watermelon: Foliar symptoms are often mild when compared to the other cucurbit
hosts. Mild leaf crinkling with some yellowing may be observed.
CONDITIONS FOR DISEASE DEVELOPMENT:
CMV can infect both greenhouse and field-grown vegetable crops. CMV has an
extensive host range (>1200 species) facilitating its survival on weeds, ornamentals,
and other cultivated crops. The primary mode of transmission is by aphids in a
non-persistent manner, although the virus can also be mechanically transmitted
through equipment and workers. Cucumber beetles (Diabrotica spp.) have also been
shown to transmit CMV, but aphids are the primary vector.
CONTROL:
Management of CMV through vector control (e.g., insecticides, stylet oils) has
been only marginally successful. Avoid planting near older cucurbits and perennial
ornamental crops, which may serve as reservoirs for the virus. Control weeds, use
reflective mulches, deep plow crop residues and dispose of infected greenhouse
material to manage this virus. Implement a comprehensive sanitation program for
workers and equipment to minimize disease spread. Commercial varieties with
resistance offer the best means to control CMV. Resistance in cucumber has been
shown to be very effective on a global scale. Progress in finding resistance in other
cucurbit species has not been as successful. In yellow summer squash, the presence
of the precocious yellow gene has worked well against CMV infection. A few
transgenic-resistant squash cultivars are grown in the USA.

CUCUMBER VEIN-YELLOWING

CAUSAL AGENT:
Cucumber vein-yellowing virus (CVYV)
VECTOR:
Silverleaf whitefly (Bemisia tabaci)
DISTRIBUTION:
Iran, Israel, Jordan, Spain, Sudan, Turkey
SYMPTOMS:
Melon and cucumber exhibit vein yellowing, vein clearing, chlorosis, necrosis and
stunting with a corresponding reduction in yield. Parthenocarpic cucumbers exhibit
severe symptoms, while non-parthenocarpic cucumbers have been reported to be
asymptomatic carriers of the virus. Cucumber fruit symptoms appear as a light to
dark green mosaic. In watermelon, foliar symptoms are inconspicuous or not expressed,
however fruit cracking has been observed. Squash may be asymptomatic or may
exhibit mild vein yellowing and chlorotic mottling of leaves.
CONDITIONS FOR DISEASE DEVELOPMENT:
CVYV is transmitted in a semi-persistent manner (<six hours) by the silverleaf whitefly,
Bemisia tabaci. Movement of infected transplants can spread the virus over long
distances. Whiteflies can easily spread the virus from plant to plant. CVYV is not considered
to be seed-borne nor seed-transmitted. The virus survives in cucurbit weeds
and volunteers, jimsonweed, Nicotiana spp., sowthistle, bindweed and Malva spp.
CONTROL:
Planting resistant cultivars is the best means of control. Currently, resistance is
only commercially available in cucumber. Implement insect exclusion (minimum
50–52 mesh/297 micron screen) to minimize whitefly infestation in greenhouse-grown
crops. Seedlings should be grown in a whitefly-free environment. Use insecticides
to control the vector. Rotate modes of action to prevent development of insecticide-resistant whitefly populations.

 

CUCURBIT APHID - BORNEYELLOWS

CAUSAL AGENT:
Cucurbit aphid-borne yellows virus (CABYV)
VECTOR:
Several aphid species
DISTRIBUTION:
Worldwide
SYMPTOMS:
Early symptoms manifest as chlorotic spots on the lower leaves, progressing to
interveinal chlorosis. Leaves become chlorotic, leathery and brittle, while the mid-vein
and primary veins remain green. Stunting and flower abortion reduce marketable
yield; however, for fruit that develop, fruit shape and quality are not affected. Prior to
the development of specific detection methods, symptoms of CABYV were often
attributed to nutrient deficiencies, senescence, or diseases, such as lettuce infectious
yellows, cucumber yellows or cucurbit yellow stunting disorder, all of which cause
similar symptoms.
CONDITIONS FOR DISEASE DEVELOPMENT:
This virus is acquired by phloem-feeding aphid vectors in a persistent manner. The
cotton melon aphid, one of the vectors of CABYV, is very efficient at transmitting the
virus. Cucurbits are the primary hosts of CABYV. Alternate hosts include crops such
as lettuce (Latuca sativa) and fodder beet (Beta vulgaris). Weeds are also recognized
reservoir hosts of CABYV.
CONTROL:
Implement an insecticide spray program to control the aphid vectors. In open-field
production, use of silver reflective plastic mulches can help to repel aphids. In protected
culture, insect exclusion (minimum 50–52 mesh / 297 micron screens) can provide
some control. Melons with aphid resistance usually significantly delay CABYV infection.

 

 CUCURBIT YELLOW STUNTING DISORDER

CAUSAL AGENT:
Cucurbit yellow stunting disorder virus (CYSDV)
VECTOR:
Silverleaf whitefly (Bemisia tabaci biotypes B and Q)
DISTRIBUTION:
Worldwide
SYMPTOMS:
CYSDV was previously thought to be restricted to the Cucurbitaceae family, but it
is now recognized that CYSDV also infects crop and weed species such as alfalfa,
lettuce, snap bean, alkali mallow and Wright groundcherry. Symptoms initiate as
interveinal mottling on older leaves, intensify with age and become systemic
throughout the plant. Veins remain relatively green as the rest of the leaf turns yellow.
Leaves may roll upward and become brittle. Melon and cucumber exhibit the most
severe symptoms, which can be confused with nutrient deficiency or other yellowing
viruses. Melon fruit does not express obvious symptoms, although sugars can be
reduced dramatically.
CONDITIONS FOR DISEASE DEVELOPMENT:
CYSDV is transmitted by the silverleaf whitefly vector, Bemisia tabaci biotypes B
and Q, which can be transported long distances via air currents. Outbreaks are often
associated with high infestations of Bemisia tabaci. The virus is not mechanically
transmitted and is not seed-borne or seed-transmitted. Bemisia tabaci needs to feed
for at least 18 to 24 hours to transmit the virus and can remain infective for up to
eight days.
CONTROL:
Insect exclusion (minimum 50–52 mesh/297 micron screens) and a preventative
insecticide spray program in transplant nurseries can help to minimize whitefly
infestations. Yellow adhesive traps are useful for monitoring the presence of Bemisia
tabaci. Control weeds to eliminate potential sources of inoculum. In open-field
production, early season exclusion of the vector using mesh tunnels may delay virus
infection. Application of insecticides for whitefly control is not an effective method to
manage virus spread in the field. Commercial varieties with CYSDV resistance are
available in cucumber, but are not yet available in other cucurbits.

 

GEMINI VIRUSES

 

VECTOR:
Silverleaf whitefly (Bemisia tabaci biotypes A, B, Q)
SYMPTOMS:
Geminiviruses infect cucurbits with varying degrees of severity. Cucumber appears
to be least affected by geminiviruses. Symptoms of geminivirus infection may include
upward curling of leaf margins, foliar stunting, chlorosis, interveinal mottling, vein
clearing and thick, distorted veins. Flowers of infected plants are small and fail to
develop normally. Early-season infection results in lack of fruit set, whereas fruit set
prior to infection may be reduced in size, and exhibit chlorotic blotches and deformation.

CONDITIONS FOR DISEASE DEVELOPMENT:
Geminiviruses are vectored by the silverleaf whitefly, Bemisia tabaci (biotypes A, B,
Q). The adult whitefly acquires the virus from infected plants and can transmit it to
healthy plants within a few hours. Symptoms can develop within five days of virus
transmission. Symptoms on cucurbits are most severe when whitefly populations
are high and the crop is infected early in the season.
CONTROL:
Host-free periods have been shown to be an effective measure for controlling the
whitefly vector, whereas insecticide spray programs have been largely ineffective.
Cultural control methods include weed control, incorporation of infected crop debris
immediately after harvest and avoidance of planting new fields near infected cucurbit
fields. Plant resistance is limited in commercial cultivars.