Viruses affecting melons
(watermelon, rockmelon and honeydew)
Christine Horlock and Denis Persley, Agency for Food and Fibre Sciences, Horticulture
Figure 1. PRSV-W affected rockmelon (left) and honeydew melon leaves (right).
Virus infection (mosaic symptoms) in Queensland melons is most commonly caused by one (or more) of three viruses, Papaya ringspot virus-type W (PRSV-W), Watermelon mosaic virus-2 (WMV2) or Zucchini yellows mosaic virus (ZYMV) all of which belong to the Potyvirus group. Potyvirus infection can result in severe losses through reduced fruit set and fruit distortion in all melon types. A fourth virus Squash mosaic virus (SqMV), belonging to the Comovirus group, has been found infrequently in melons in Queensland. This virus is seed-borne and can also be spread by leaf eating beetles.
Papaya ringspot virus (previously Watermelon mosaic virus type 1) occurs as two strains. Type W (for watermelon) infects only cucurbits (e.g. squash, pumpkin, cucumber and melons) while type P (for papaya) infects both papaya and cucurbits. The two types are very closely related, except for the inability of type W to infect papaya.
Symptoms
The most characteristic symptom of virus infection in melons is a mosaic pattern on the leaves (Figure 1). The three viruses cause similar symptoms, and mixed infections are common, hence it is usually difficult to give a definitive diagnosis based on symptoms alone (Figure 2).
Papaya ringspot virus-type W (PRSV-W)
PRSV-W can cause a prominent light-green and dark-green mosaic pattern on melon leaves. Terminals of recently affected watermelons tend to stand more erect, with the mosaic pattern developing later. In the later stages of more severe infections, the crown of the plant appears to die back, while the outer runners maintain relatively green leaves. Watermelon fruit can develop distinctive ringspot patterns on the skin Figure 3). Alternatively, watermelon fruit skins can develop a roughened texture, ranging from small "pimple-like" lumps to a distinctly knobbled appearance with a yellow mottled pattern. Rockmelon fruit affected by PRSV-W are often poorly netted, while honeydew fruit can fail to ripen.
Figure 2a. From left to right, squash leaves affected by PRSV-W, WMV2 and ZYMV.
Figure 2b. From left to right, squash leaves affected by SqMV.
Figure 3. PRSV affected watermelon fruit displaying distinctive ringspot patterns on skin.
Zucchini yellows mosaic virus (ZYMV)
ZYMV induces extreme symptoms on all melon types, including severe yellow mosaic on leaves, which is usually associated with distortion, deformation and blistering of fruit. Plants are often stunted with poor fruit set. The first few leaves affected on rockmelons may progress from chlorosis to full necrosis. A number of ZYMV variants have been described, some strains may induce lethal wilting, others can cause cracks on the fruit, or mosaic and hardening of the fruit flesh.
Watermelon mosaic virus 2 (WMV2)
WMV2 generally produces less severe symptoms than either PRSV-W or ZYMV, causing mosaic symptoms on leaves, but rarely fruit distortion in melons. Symptoms produced by WMV2 are probably the most variable (between melon cultivars) of the four viruses mentioned, with some varieties showing very severe vein banding, green mosaic, deformation and plant stunting, whilst others are almost symptomless. WMV2 can also produce ringspot patterns on watermelon fruit skin. Interestingly fruit affected by WMV2 in this way can ripen normally, and contain good quality flesh.
Distribution and importance
PRSV-W epidemics are most common in tropical or subtropical areas where the wild cucurbit species act as reservoirs of the virus and where cucurbits are able to grow throughout the year. PRSV-W's host range is restricted to cucurbitaceous species, and it can only infect either commercial (or home grown) cucurbits or native cucurbit species (ie: prickly paddy melons, wild gherkins, "pie" and "jam" melons). Since the 1970's, PRSV-W has become widespread in all the major melon growing areas in Queensland. PRSV-W has previously been prevalent in coastal production areas, causing major yield losses, by severely reducing fruit set and plant vigour in all melon types.
ZYMV was first identified in Italy and France in 1981, and spread within a decade to major cucurbit producing regions worldwide. In Queensland, ZYMV occurs in most coastal production areas (eg: Bundaberg, Burdekin, Lockyer Valley and Fassifern Valley) and is a major cause of loss. ZYMV has not been detected in inland production areas (eg: Biloela, Chinchilla and Emerald). The economic effect of the virus can be dramatic, with early infections generally leading to a complete yield loss.
WMV2 is widely distributed throughout the world. WMV2 occurs throughout Queensland, infecting a wide range of commercial cucurbits and native cucurbit types. However, in Queensland WMV2 infection does not generally cause major losses, and it is currently the least important of the three potyviruses.
Source of infection and spread
For an epidemic of WMV2, PRSV-W or ZYMV to occur, there needs to be virus-infected source plants, winged aphids and susceptible host plants. Disease outbreaks occur when winged aphids feed on virus-infected source plants, fly to healthy plants and transfer the virus by feeding again. The viruses do not survive in the soil or in decaying plant material.
Infected source plants
The main sources of infection for the three viruses are old, diseased cucurbit crops, cucurbits in the home garden, and weed species such as volunteer melons and native cucurbits (ie: prickly paddy melons, wild gherkins, "pie" or "jam" melons)(Figure 4). Infected plants survive from one cropping cycle to the next to act as source plants for the following season. Virus-infected source plants must also be healthy enough to appear appetising to aphids. PRSV-W and WMV2 are not carried in seed and only an extremely low rate of seed transmission has been found with ZYMV.
Figure 4. WMV2 affected "pie" melon, found in a commercial rockmelon crop.
Aphids
PRSV-W, WMV2 and ZYMV can be spread in a non-persistent manner by many species of aphid moving through or within a crop. Non-persistent viruses are acquired and transmitted by aphids when they are briefly probing plants. These viruses may be acquired and transmitted by aphids feeding on leaves for 10 seconds or less, with the insect remaining capable of transmitting virus for up to several hours after feeding on an infected plant.
How many aphids do you need to have an epidemic? Winged aphids may be carried several kilometres by wind, which often occurs when migrating aphids are seeking their preferred host species. Thus, given that the vector aphid may come from elsewhere, and can transmit the virus during very brief probes (2-10 seconds), small numbers of aphids are able to successfully transmit these viruses to large numbers of plants. Consequently, devastating crop losses can occur without significant numbers of aphids being observed.
An aphid species that doesn't like the crop can often do the most damage. The main aphid vector of a potyvirus is often a species that does not normally settle and reproduce on that crop; potyviruses are transmitted by probing aphids flitting between plants, and so anything that keeps them on the move will enhance transmission.
No other insect species has been found to transmit PRSV-W, WMV2 or ZYMV. Vector transmission is usually a very specific property of each virus. All potyviruses that have been tested by scientists are transmitted in nature, and in experiments, by aphids. Potyviruses have not been transmitted with other 'plant bugs', such as leaf hoppers or beetles. The transmission process is a biological one, not mere contamination of the aphids mouth parts by the virus. The potyviruses adhere to the aphids stylets while probing and are then released during the next probe.
Other host plants as sources of virus
The exact role of other host species in the potyvirus disease cycle remains unclear. Certainly, all three viruses can also infect other cucurbits including zucchini, pumpkin, cucumber and squash. PRSV-W and WMV2 resistant varieties of zucchini and pumpkin, grown as commercial crops can act as a very efficient reservoir of virus, as plants become infected and propagate the virus, without showing any symptoms.
The large population of wild / native cucurbit species such as prickly paddy melons (Cucumis myriocarpus), wild gherkins (Cucumis anguria), and "pie" or "jam" melons (Citrullus lanatus) which grow throughout Queensland also have the potential to act as reservoirs of cucurbit viruses. Many symptomless wild melon plants have been found to be infected with both WMV2 and PRSV-W.
WMV2 also infects a wide variety of plant species outside of the cucurbit family, including solanaceous and asteraceous plants as well as cucurbits. PRSV-W, on the other hand, infests only members of the cucurbit family. ZYMV has only been found on cucurbits in Australia.
Management
PRSV-W, WMV2 and ZYMV are all transmitted in the same manner and can all be managed using the same techniques. Although individual plants cannot be "cured" of virus infections, it is possible to improve crop yields and produce quality fruit with careful management. Focus anti-virus strategies on the first half of the crop's life. The greatest yield losses occur if plants are infected prior to flowering and fruiting. If virus infection occurs after fruit set then yields may not be significantly affected.
Breaking the disease cycle
Virus control focuses on breaking the disease cycle by preventing or reducing virus movement from infected plants, through aphids, to healthy plants. If at all possible, avoid overlapping crops of cucurbits - especially if a certain planting is already known to be infected. Try to have a part of the year (off-season) when no cucurbits are grown, and cucurbit weeds and volunteers are removed entirely.
Farm hygiene
Plough in old cucurbit crops as soon as harvesting is completed. Old, badly diseased plantings are the major source of virus for new plantings. Try to arrange sequential plantings so that younger plants are upwind of older crops. Keep crop and surrounding areas free of weeds, particularly native cucurbits and volunteer melons.
Aphid control
Super-reflective plastic mulch deters aphids from landing on leaves, and subsequently feeding on plants. These mulches can be highly effective, even when partially covered by growing plants. Remember the first few weeks (until flowering) is the time when virus infection will do the most damage. This treatment may be used in conjunction with weekly applications of stylet oil which is applied to cover leaf surfaces thoroughly.
Insecticides
Virus levels in melon and other cucurbit crops are often very high, despite regular applications of insecticides to prevent aphids colonising and reproducing on plants. This situation is more easily understood when it is realised that the main aphid vector of a potyvirus is often a species that does not usually settle and reproduce on the crop. The most efficient aphid vectors are aphids flitting between plants, making brief probes as they move, and transmitting virus as they briefly probe plants searching for a suitable host. Thus, given the aphid may come from elsewhere, and best transmits virus during very brief probes (2-10 seconds) on leaves, it is not surprising that treating a crop with insecticide will have no effect on the spread of potyviruses. In some experiments insecticides have increased virus spread by disturbing the aphids before death occurs.
Economic cropping
Consider having a shorter melon production season, only growing melons when the price is good, and you can produce high quality fruit. Alternatively, have a break in production (with no cucurbits in the ground) if a long season is planned, and virus incidence is high. In either case by growing melons for shorter periods there is less opportunity for large amounts of virus to build up in both crops and surrounding weeds. Minimise virus management expense by using expensive anti-virus techniques (such as reflective mulch) only at times of the year when virus infection is likely.
Further information
Other DPI melon disease notes:
Bacterial fruit blotch of melons (watermelon, rockmelon and honeydew)
Fusarium Wilt of melons (watermelon, rockmelon and honeydew)
Powdery mildew of melons (watermelon, rockmelon and honeydew)
Sudden Wilt of melons (watermelon, rockmelon and honeydew)
Key contacts
Denis Persley
Plant Pathologist, Horticulture
Agency for Food and Fibre Sciences
Department of Primary Industries
Indooroopilly Research Centre
80 Meiers Road
Indooroopilly Qld 4068
Phone: (07) 3896 9375
Fax: (07) 3896 9533
Email: denis.persley@dpi.qld.gov.au
Christine Horlock
Plant Pathologist, Horticulture
Agency for Food and Fibre Sciences
Department of Primary Industries
Applethorpe Research Station
New England Highway
PO Box 501
Via Stanthorpe Qld 4380
Phone: (07) 4681 1255
Fax: (07) 4681 1769
Email: christine.horlock@dpi.qld.gov.au
Telephone the DPI Customer Service Centre on 13 25 23 (Queensland residents) for the cost of a local call between 8 a.m. and 6 p.m. weekdays; non-Queensland residents phone 07 3404 6999; email callweb@dpi.qld.gov.au.
Infopest CD for current registered pesticides: email infopest@dpi.qld.gov.au.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: H0233
Last reviewed 21 January 2004