Cucumber mosaic virus (CMV) is a prevalent viral disease of cucurbits, and is spread by 80 or more species of aphids in a non-persistent manner. Non-persistent viruses are not retained inside the vector for a long period of time, but are rapidly transmitted to a host, leaving the vector virus-free. CMV remains in the stylet of the aphid, and is deposited into a new potential host upon feeding.
Other important cucurbit viruses transmitted via aphid vectors include watermelon mosaic virus, zucchini yellow mosaic virus and papaya ringspot virus. These differ from CMV, with a much more limited host range – CMV can affect over 1,200 species of plants – and some differences in genetic makeup. All can cause similar symptoms in cucurbits, including the signature mosaic of mottled leaves with light and dark green patches, and deformed fruits.
But growers can’t count on these symptoms alone to diagnose virus diseases.
“The mosaic symptoms observed could result from a rather large number of different viruses, some of which are not transmitted by aphids,” said John Murphy, Ph.D., Entomology and Plant Pathology, Auburn University. “A good example is squash mosaic virus, transmitted by beetles and through seed.”
Another complication is mixed infections, which “occur very commonly in the field, or within a single plant,” he added.
One virus can mask the symptoms of others or can synergistically increase symptoms. Plant species, variety and crop age can all cause various changes in symptoms, too. Often, it isn’t necessary to identify the exact viruses causing problems in a field. Recognizing the existence of possible vectors, and taking precautions to reduce their impact, is the best defense.
Viruses can’t be controlled with any antimicrobial sprays. Crop susceptibility and vector control are two important areas of focus.
“One issue associated with focused efforts to manage aphid-borne viruses is their sporadic nature,” Murphy said. “Incidence can swing from very low to high to back to low over several seasons. This is due, in part, to the need for sources of virus and vector to coincide with a vulnerable crop.”
Planting virus-resistant crops is one option for protecting against any major viral attacks. Resistance has a range, falling between fully susceptible and full immunity. Tolerance indicates a degree of resistance closest to immunity, while resistance means that the plant is susceptible, but has mechanisms keeping the virus in check, so it can’t replicate freely, and less viral load is accumulated.
“True tolerance means that the virus infects the plant, accumulates and moves cell to cell as if it is susceptible. It moves throughout the plant as if it is a susceptible plant, and it accumulates freely throughout all the tissues, but that plant is able to tolerate infection” with no more than minor symptoms and no effect on the fruit, Murphy said.
While tolerant varieties seem like a good idea, and do have positive results in the short term, they may actually have a negative long-term impact. Virus loads could build up in the fields and ultimately inoculate neighboring crops or nearby weeds, which also serve as virus hosts.
Growers may turn to insecticidal sprays to target aphids. However, it typically isn’t the aphids seen colonizing the crop, but transient aphids, which spread most of the virus. Insecticides can also compound the problem, causing aphids to move more rapidly from plant to plant, particularly if the spray does not have an immediate effect, according to Murphy.
A reliable method of retaining control of viruses is to delay their introduction to the crop. Because mature plants are less susceptible and show milder symptoms, keeping aphids out of the young crop can be an effective way to decrease virus concerns.
Silver reflective mulch has been found to reduce the population of aphids moving into a field. It is less effective if aphids are already present.
“The use of UV-reflective mulch can significantly reduce losses by insect-borne viruses and from many insect pests,” he said. “This, in combination with timely applications of insecticides, can be very effective.”
Another alternative is to plant inter-row living ground covers. Aphid behavior is to test plants by probing them before committing to feeding. If plants other than the cucurbit crop could induce this behavior, then the virus would be deposited in their tissue, and not the crop. Additionally, insects don’t see their target food source and won’t be enticed to land when everything is a green. Bare brown soil actually attracts pests.
“Inter-row crops should not be a host of the virus, and you certainly don’t want to attract the vector,” Murphy said.
Applying biological preparations is another option that involves one or more bacterial species, applied through plant roots or via seed treatments. These interfere directly with pathogenic viability, provide minerals and nutrients to the plant to enhance growth and or provide systemic resistance to disease. Murphy has had success reducing CMV via biological preparations in research trials.
Crop rotation, too, can play a role. As viral load builds within a crop, the vector can spread the virus to other susceptible nearby plants. These plants may then serve as a viral source when the insect vector returns the next season. Rotating cucurbits can decrease the likelihood of subsequent infection from one year to the next.
“For insect-borne plant viruses, crop rotation could serve to break up the buildup of virus inoculum, vector populations or both if the rotated crop is not a host for either,” Murphy said.
A large source of virus, vectors and a vulnerable crop are all that’s needed to spread a virus.
Aphids are primary vectors for many cucurbit viral diseases. Managing the aphid population, which transmits CMV and other cucurbit viruses in a non-persistent manner, requires a combination of strategies aimed at interrupting the virus-vector-host relationship.
“You really need to know your virus-host system,” Murphy said. “Virus-vector relationship is very specific to that virus and that vector.”
Murphy recently presented a webinar on the topic that can be viewed online.