Jerry Brust, IPM Vegetable Specialist, University of Maryland; email@example.com
Cucumber mosaic virus (CMV) has been found in some watermelon fields in central Maryland. CMV is not uncommon in the NE United States, but in a two-year survey of pumpkin that Kate Everts, Karen Rane, Mark VanGessel and I did in Maryland and Delaware we did not detect CMV in any pumpkin field we sampled. CMV is transmitted primarily by aphids, but also by cucumber beetles, mechanically and to a lesser extent in seed. Many species of aphid can vector the virus in a non-persistent manner – the most common species in Maryland are: Melon aphid Aphis gossypii and Green peach aphid Myzus persicae. The virus is acquired by aphids within 10 seconds after they begin to probe an infected plant. The virus can be transmitted to other plants by aphids in less than one minute. This is why insecticides do not stop initial infections. Aphids lose the ability to transmit CMV after about 2 minutes and completely lose the ability after 2 hours. There are many strains or types of CMV, some isolates can lose their transmissibility by one aphid species but retain their transmissibility with another. In one field I visited there were many early season striped cucumber beetles and I think they may have been responsible for transmitting the virus from weeds to the watermelon in this field. The virus symptoms were first seen a few weeks ago – very early in the season for us to be seeing virus symptoms in watermelon fields. Some of this may be due to early season cucumber beetles transmitting the virus and some may be due to the extreme heat and drought we are experiencing. The drought is causing weeds to wither and the aphids that are present on them are moving earlier than they normally would to greener fields—like our well cared-for cucurbit fields.
The symptoms I have seen on watermelon are rather non-descript (Fig. 1) and look like they could be due to many things, including herbicide injury. Leaves of new growth are crinkled and deformed with a slight yellowing to them. CMV produces a systemic infection in most host plants with the older plant tissues that developed before infection rarely being affected by the virus. Tissues that develop after infection are affected to varying degrees. The concentration of the virus increases for several days following inoculation, and then decreases until it levels off. While there is transmission through seed in 20 host species, the most important source of virus may be weeds, which allow for overwintering of the virus. In addition to seeds, CMV overwinters in many perennial weeds and some crop plants. The perennial weeds wild ground cherry, horse nettle, milkweed, ragweed, pokeweed, nightshade, and various mints can harbor the virus in their roots, and in the spring the virus migrates to new growth, which aphids then transmit to susceptible crop plants. CMV is easily transferable through sap carried on the hands, clothes and tools of people harvesting fruit, weeding or turning vines in a watermelon field.
Figure 1. New growth on watermelon with CMV symptoms
There are some CMV resistant (tolerant) cucumber varieties available that produce a good crop, but most other cucurbits are susceptible to CMV. Using reflective mulch reduces the early season infection from aphids and gives an additional 2-4 weeks of a virus-free cucurbit field. Once the plants cover the plastic the reflective mulch ceases to be an effective deterrent. Pesticides only work to reduce the in-field spread of aphids and therefore, CMV and other viruses.
I have also seen virus symptoms in cantaloupe recently. Leaves are mottled and puckered (Fig. 2), and symptomatic tissue tested positive in a generic potyvirus test. Initial tests for the common potyviruses Watermelon Mosaic Virus, Zucchini Yellow Mosaic Virus and Papaya Ringspot Virus were negative for these cantaloupe leaves. Additional tests are being performed to try to identify which specific potyvirus is affecting the cantaloupe.
Figure 2. Cantaloupe with potyvirus symptoms
In addition to the virus problems, I have seen, especially in the last two weeks, a great increase of necrotic spot appearing on the crown leaves of watermelon plants (Fig. 3). This is being caused by a combination of air pollution problems-hot humid air not moving much and the fact that the plant takes nutrients from the crown leaves when there is a heavy demand from the growing fruit. This results in the crown leaves turning brown and eventually breaking down. In cantaloupe I have seen some leaf marginal chlorosis (yellowing of leaf margins) of some leaves (Fig. 4). This is often caused by salt deposits (from foliar nutrient or pesticide sprays) that accumulate around leaf margins, which have a toxic effect on the gas exchange pores (hydathodes) located at leaf tips. This is commonly seen under high temperature and humidity.
Figure 3. Necrotic spots on watermelon crown leaf
Figure 4. Marginal chlorosis of cantaloupe leaf