Squash Mosaic Virus Background

Squash Mosaic Virus (SqMV)
Family: Comoviridae
Genus: Comovirus
Vector: Seed, Beetles

Squash Mosaic Virus is one of several viruses causing mosaic diseases of cucurbits. SqMV affects most cucurbits, occurring naturally in squash, cucumber, pumpkin, melon and gourds. Other plants infected by SqMV include garden and sweet peas, coriander, and salad chervil. SqMV is not generally found to affect watermelon. First identified in California in 1934, SqMV has been reported in CA consistently for the last 30 years.

Of the several key cucurbit diseases, SqMV is the only one that is seed-transmitted. The pathogen is permanently preserved and carried by infected seed, and cannot be eliminated by typical hot water or chemical treatment with tusodium phosphate. As a result, most of the outbreaks of this virus can be traced to infected seeds. Uncontrolled introduction of infected seed has greatly facilitated its spread. While SqMV was only recently found in New Zealand - by way of seed imported from the US – it is a worldwide problem.

Planting of infected seed is considered the primary source of inoculum; seed transmission has been reported ranging from 1 to 10%, though much higher percentages have been recorded. Plants grown from infected seed serve as inoculum for secondary disease cycles. From plant to plant, SqMV is efficiently spread by the striped and spotted cucumber beetles (Acalymma spp. and Diabrotica spp.). Cucumber beetles can become infective after feeding on an infected plant for only five minutes and can then transmit the virus from plant to plant for 4 to 20 days. SqMV survives between vine crop plantings in infected weed hosts, in infected seed, and in overwintering beetles.

SqMV is one of several cucurbit viruses with similar symptoms. SqMV-infected plants may show ring patterns, severe blister mottle, deformation, and occasionally enations. Infected plants may also be symptomless. Fruit symptoms range from small chlorotic areas to severe deformation with raised dark green areas. It is impossible to identify SqMV with certainty based on symptoms alone; special diagnostic testing is required to correctly distinguish it from other cucurbit viruses.

SqMV causes severe systemic infection often resulting in economic damage and crop loss. It can cause considerable loss in late-season squash and muskmelon crops, and in the case of early infection, fruits may be deformed and unmarketable. According to the USDA, cucurbits have an annual value of more than $2.2 billion to U.S. growers. While controlling the disease is difficult,

“Effective management of these diseases is often through the use of certified pathogen-free seed lots. However, current individual seed health testing methods (e.g., seedling grow-out, culturing or ELISA) are cumbersome and expensive.”

EnviroLogix' new QualiPlate Kit for SqMV has overcome this hurdle by providing a simple yet accurate ELISA testing protocol - with a 2-hour or 24-hour time frame – all with a cost of less than $1 per test.

Despite many years of breeding research to develop SqMV-resistant varieties, success has been elusive. The Cornell Research Foundation (Ithaca, NY) was granted a patent for “transgenic plants that contain squash mosaic virus coat protein genes that are resistant to infection by squash mosaic virus.” This and other genetic and biological breeding programs focusing on developing resistance to SqMV and other diseases in cucurbits is ongoing – if not accelerating.

Squash Mosaic Virus symptoms on mature melon leaves appear as intense dark green mosaic, with blistering and hardening of leaf texture.	Melon fruit infected with Squash Mosaic Virus show strong mottled patterns and lack of netting.