Viral Diseases

MELON NECROTIC SPOT
POTYVIRUSES
SQUASH MOSAIC
SQUASH VEIN-YELLOWING
TOBAMOVIRUSES
TOSPOVIRUSES

MELON NECROTIC SPOT

CAUSAL AGENT:
Melon necrotic spot virus (MNSV)
VECTOR:
Olpidium bornovanus
DISTRIBUTION:
Worldwide
SYMPTOMS:
This virus has a narrow host range, affecting only melon, cucumber and watermelon.
Symptoms initially appear on younger leaves as chlorotic spots, which soon become
necrotic. In some cultivars, necrotic lesions and streaks develop on petioles and
stems, indicating systemic infection. Leaves may curl and wilt, occasionally leading
to complete plant collapse. Symptom intensity may vary considerably depending on
cultivar. In watermelon, MNSV manifests as local lesions only.

CONDITIONS FOR DISEASE DEVELOPMENT:
MNSV is transmitted by zoospores of the obligate fungal parasite, Olpidium
bornovanus. MNSV has been shown to be seed-transmitted at low rates. This virus
can also be mechanically transmitted by pruning, workers and equipment. Symptoms
develop mainly under cool, low light conditions. In summer, infected plants may show
no symptoms.
CONTROL:
Grow resistant cultivars, incorporate plant debris, fumigate soil and rotate out of
cucurbits to manage this virus. Solarization has also been effective in favorable
environments. Avoid excessive irrigation to minimize spread of the fungal vector.
Surfactants added to irrigation systems may reduce zoospore numbers, thus
minimizing spread of the vector in soil-less media. Grafting watermelon onto
MNSV-resistant rootstocks is an effective cultural practice to control MNSV.

 

 

POTY VIRUSES

CAUSAL AGENTS:
Moroccan watermelon mosaic virus (MWMV)
Papaya ringspot virus (PRSV; formerly Watermelon mosaic virus-1)
Watermelon mosaic virus (WMV; formerly Watermelon mosaic virus-2)
Zucchini yellow mosaic virus (ZYMV)
VECTOR:
Aphis spp.
DISTRIBUTION:
PRSV, WMV, ZYMV – Worldwide
MWMV – Africa (Morocco, South Africa), France, Italy, Spain, and Portugal
SYMPTOMS:
All cucurbits are susceptible to potyviruses. Due to the similar nature of symptoms
caused by the various potyviruses, it is recommended that samples be submitted to
a diagnostic laboratory for proper identification. A characteristic symptom common
to all curcubit polyviruses is a narrowing of tendril-like appearance of leaves, referred
to as “shoe-string.”
PRSV: Symptoms can initially appear as vein-clearing of leaves. As symptoms
progress, a light to dark green mosaic develops, followed by distortion and deep
leaf serration. In cucumber, leaves tend to be distorted along the margins. In melon,
severe infection can cause blistering of young leaves. In squash, severely affected
leaves can assume a “shoe-string” appearance. In watermelon, the growing terminals
tend to stand erect and new leaves are reduced in size. Early-season infection may
lead to poor fruit set while late-season infection may result in deformed, blotchy fruit.
Concentric ring-spot patterns may develop on the rind of watermelon fruit.
WMV/MWMV: Symptoms manifest as chlorosis of leaf veins. As the disease
progresses. Leaves may develop a green mosaic and become deformed and
blistered. In severe cases, leaf tissue surrounding the major veins develops a “shoestring”
appearance. Early plant infection often leads to severely distorted, discolored
fruit. When virus infection occurs after fruit set, generally fruit development is normal.
MWMV causes very severe mosaic and deformation of leaves and fruits in cucumber, squash and watermelon. In many melon cultivars, systemic infection manifests as

necrotic leaf spots, which is often followed by complete plant collapse. MWMV is
almost exclusively restricted to cucurbits, whereas WMV has the widest host range
among the potyviruses.
ZYMV: Infected leaves are yellow with severe mosaic symptoms and also may exhibit
blistering and “shoe-string.” Early-season infection can cause stunting of plants,
uneven fruit color and fruit malformation.
CONDITIONS FOR DISEASE DEVELOPMENT:
All potyviruses are vectored in a non-persistent manner by several species of aphid.
These viruses can also be mechanically transmitted by workers and equipment to a
lesser extent. The host range for some of these viruses includes legumes and weeds,
however, infected weeds may remain asymptomatic.
CONTROL:
Grow resistant cultivars, control aphids and weeds and avoid planting near older
cucurbit fields. Reflective mulches, equipment and worker sanitation, deep plowing
of crop debris and destruction of cull piles may also help control these diseases.