Buttercup squash viruses in New Zealand
J. D. Fletcher and T. J. B. Herman

Apart from outbreaks of watermelon mosaic virus (WMV2) and squash mosaic virus (SMV) in the early 1970s, New Zealand squash crops have been remarkably free of virus diseases until recent years.

Zucchini yellow mosaic virus (ZYMV) was isolated from squash crops in Hawke’s Bay in the 1994/95 season, coinciding with a resurgence of WMV2. These viruses cause leaf mosaic and fruit damage. Subsequent research has shown that infection with either of the viruses could reduce fruit number (up to 15%), reduce the weight of marketable fruit (up to 85%) and reduce the total weight of fruit (up to 47%). Over the following seasons, the incidence of ZYMV and WMV2 varied in timing and severity.

The Viruses
ZYMV and WMV2 have the potential to infect any cucurbit crop currently grown in New Zealand. ZYMV has spread rapidly throughout the world over the past ten years, but in New Zealand it is found only in Hawke’s Bay. ZYMV is found only on cucurbit crops while WMV2 can potentially infect a range of other crops including peas, beans, lupin, phacelia and clover.

ZYMV has been recorded in 13 weed species and WMV2 in 26 weed species these are listed, with associated aphids in Table 1. (click here to view Table 1). In both cases the weeds are common in and around New Zealand crops and could provide alternate, overwintering hosts for the viruses.

SMV is primarily seed-borne and has been seen only rarely in New Zealand in recent years because of good quality control during seed production. The virus spreads only slowly in a crop, mostly from mechanical transmission.

Which virus is in my crop?
It is common for mixtures of these viruses to occur not only in your crops but also in individual plants. The symptoms of each virus, listed below, can also differ according to cultivar and time of infection.
You must, therefore, be careful when making your own diagnosis of which viruses are present in your crop and you should consider having your diagnosis confirmed (see below).

ZYMV:
yellow vein-banding, chlorotic yellow mosaic with distortion and blister of leaves; sometimes “fan leaf” occurs; and in mature crops, yellow water shoots may sometimes stand out above the crop.
Plants may be stunted. Fruit symptoms include pimpling, blisters and reduced size. [View Fig 1] [View Fig 2]

SMV:
chlorotic yellow spotting of the first leaves, severe spotting, distortion and sometimes necrosis of later leaves.
Plants may be stunted and fewer fruit may be produced. [View Fig 4]

How do viruses spread into and around my squash crops?
The most common squash viruses, ZYMV and WMV2, are primarily transmitted from plant to plant by aphid vectors. Aphids can become infected after feeding for only 15 seconds on an infected plant. They can subsequently transmit the virus for up to four hours.

It is likely that the first three aphids; melon aphid, potato aphid and green peach aphid, are the main vectors of squash viruses because these species also form colonies in squash crops.

ZYMV and WMV2 may also be spread through crops by mechanical movement of equipment (harrows, irrigators, or sprayers) or by people pruning and training vines. Seed transmission of ZYMV may occur at very low rates in zucchini and there is experimental evidence of seed transmission of both viruses in buttercup squash.

SMV is initially transmitted by infected seed, and, during crop growth, by mechanical spread.

How can I control the squash viruses?
A range of control methods are available to manage squash virus diseases. Aphid vectors are important in the spread of viruses into a crop and controlling these vectors is an important aspect of squash virus control.

Early detection and control is important. If crops remain disease free, especially up to flowering, fruit damage can be minimised.

During the growing season:
Control of virus diseases begins with minimising virus sources in and around the fields. Weeds that have potentially hosted the viruses during the winter should be eradicated before the new season’s crops are planted. The weeds may also host aphid vectors over winter. Good weed control will help reduce the number of aphid vectors moving into your crops and may reduce the proportion of vectors actually carrying the viruses.

Insecticides (seed treatments or foliar sprays) have little effect on preventing virus transmission, but they may reduce the spread of the virus by reducing the number of aphids.

You should choose your cultivars and seed supplies carefully. If resistant cultivars become commercially available these should be used. Only high quality, disease-free seed should be sown.

Once the crops are growing you should inspect them regularly and rogue out any plants showing virus symptoms, remove them from the field and destroy them. The movement of equipment and workers through infected crops should be restricted, especially on cool damp days, and when working in crops, always move from ‘clean’ crops to infected crops to minimize disease spread.

Equipment should be hosed clean and disinfected (0.01 % bleach or a stronger detergent) when leaving an infected crop. If pruning or training plants you should regularly wash hands and disinfect cutters, e.g. every ten plants or between rows.

Post season control
Once the crops are harvested, promptly feed off remaining fruit and vines to stock and bury any remaining trash through deep cultivation.

Avoid successive cucurbit crops in the same field, particularly if the crop was infected with the viruses.

Finally, all weeds and volunteer squash that emerge after the crop has been harvested should be killed to minimise carry over to the next season.

Research results: the importance of weeds and other virus disease risk factors.
Weeds continue to be important reservoirs of squash viruses, particularly in the Hawke’s Bay. Mosaic viruses were found in 32 weed species in and around squash paddocks. Table 1 summarises weed hosts we have recorded for WMV2 and ZYMV from surveys over past seasons. Viruses may survive in weeds and are transmitted by aphids to nearby squash crops during spring and through the growing season. Important weed families include Amaranthaceae, Solanaceae and Chenopodaceae. For example there is an association (r = 6.02; p<0.05) between the presence of mosaic virus and the occurrence of Amaranthaceae and Solanaceae (r = 7.75; p<0.05). Data collected over three seasons also reveal a correlation (r = 0.41) between the presence of mosaic and previous mosaic infections for early sown squash crops. Similarly there is evidence of an association (r =5.65; p<0.05) between mosaic virus and shelter trees; shelter was present in 59% of crops with mosaic but only 38% of crops without mosaic.
Mechanical transmission is important in subsequent disease spread from aphid initiated infections. Transmission may occur from movement of irrigators, sprayers or during inter-row cultivation.