The leaf symptoms of ZYMV infection are severe mosaic, deformation, blistering and reduced size. Infected plants are stunted. Fruit symptoms of pumpkin, zucchini, squash, watermelon and cucumber include mottled skin, uneven colouring and knobbly areas that cause prominent deformations. Infected rockmelon fruit often have poorly formed surface ‘netting'. Infected fruit has reduced shelf life. Symptoms are similar to those caused by infection with papaya ringspot virus and watermelon mosaic virus.
Sources of virus
The virus needs living plants to survive and cannot live in soil or dead plant material. ZYMV infection is usually confined to plants in the cucurbit family. The main sources of ZYMV are old diseased cucurbit crops, volunteer or self-sown cucurbits and cucurbit weed species such as Afghan or paddy melons. ZYMV also infects native cucurbit species including Cucumis maderaspatanus. These plants are found growing within crops, on roadside verges and along fence-lines. Infected old crop plants, weeds and native cucurbit species allow the virus to survive between growing seasons. ZYMV is spread from these infected plants to young crops by aphids.
ZYMV can be transmitted at very low levels (less than 1%) in the seed of zucchini and pumpkin.
Aphid vectors and transmission
ZYMV is primarily spread by aphids, including melon (Aphis gossypii) and green peach (Myzus persicae) aphids. Aphids which live on other plants but only migrate through cucurbit, such as the cowpea aphid (Aphis craccivora), can also transmit the virus. ZYMV is transmitted non-persistently. This means an aphid picks up the virus within 1-2 seconds while probing an infected plant and then transmits the virus within 1-2 seconds to healthy plants. After the aphid has probed one or two healthy plants the virus is lost to the aphid until it probes another infected plant.
A small number of aphids are able to spread the virus to a large number of plants in a short time as they search for a suitable host plant to colonise.
ZYMV is readily spread between plants when footwear, cutting implements or machinery damage infected leaves and the infective sap then brushes or rubs onto healthy plants. The virus can survive on surfaces, including metal, plastic, rubber and cotton for several hours. Such virus spread can be reduced by using footbaths or cleaning cutting implements and machinery with a 1:4 dilution of household bleach, one per cent Virkon®, or a 20% solution of skim milk powder.
Yield and quality losses
When cucurbit plants become infected early (before flowering) yield losses can be up to 100%. If infection occurs before or at fruit set then most fruit will have quality defects and be unmarketable. When plants are infected after fruit set, yield and quality losses are often lower. The severity of symptoms and magnitude of yield and quality losses depends on the time of infection, strain of the virus and the variety grown.
To minimise ZYMV spread in cucurbit crops an integrated disease management approach is necessary.
- Remove and destroy old cucurbit crops immediately after the final harvest – to minimise virus spread to new crops.
- Destroy any wild or self-sown cucurbit plants and weeds before planting – to reduce any potential virus and aphid sources for new crops.
- Remove any cucurbit plants showing virus symptoms, particularly before fruit set – removing virus sources within the crop may help to slow down the spread of the virus to nearby plants.
- Monitor surrounding weeds and crops for aphid populations.
- Plant a tall non-host border crop around the cucurbit crop about four weeks before planting – a non-host border acts as a cleansing barrier for aphids. Infective aphids that feed on it will lose the virus and will no longer be infective when they land on the cucurbit crop.
- Plant new crops upwind from older crops, avoid sequentially planting downwind – there is less infection upwind from infection sources as aphids can be blown along with the wind.
- Employ good hygiene practices - Use 1:4 dilution of household bleach or one per cent Virkon® for footbaths and to wash equipment and machinery.
- Avoid moving machinery, equipment and workers from old crops to new ones – to minimise spread from older to young crops.
- Use virus tolerant cucurbit varieties when available .
It is important to note that although ZYMV is spread mainly by aphids, insecticide use is ineffective as a control measure because they do not work fast enough to prevent aphids from feeding on and infecting a healthy plant before it is killed.