Circular Circ-1184 Aphid-Transmitted Viruses of Cucurbits in Florida Tom Kucharek and Dan Purcifull, Respectively Professor and, Professor, Plant Pathology Department, University of Florida, Gainesville, 32611. April 1997 (Revised 2001) Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean Cucurbit crops (Cucurbitaceae, cucumber & been found in Florida. These viruses are com- gourd family) are of major importance to the posed of a ribonucleic acid core encapsulated state of Florida and its agricultural communi- by a protein “coat”. Table 1 summarizes the ties. For the 1994-1995 marketing year, water- current nomenclature of these viruses. melons, squash, and cucumbers were valued at $146,079,000 in Florida. This was 9.9 percent Table 1. Aphid-transmitted viruses found of the total value of vegetables at the farm-gate naturally in Florida-produced cucurbits. and does not include other cucurbits such as Virus Acronyms cantaloupes and pumpkins. Florida produces Paypaya ringspot virus PRSV-W(WMV 1) more watermelons than any other state. In ad- type W dition to several fungal diseases, viral diseases Watermelon mosaic virus WMV 2(WMV) have strongly interfered with the establishment 2 of a pumpkin industry in Florida. Zucchini yellow mosaic ZYMV virus For at least the past three decades, the most Cucumber mosaic virus CMV important impediment to consistent production of squash, watermelons, and some other cucur- Unnamed potyvirus none bits in Florida, has been the annual occurrence of aphid-transmitted viruses. Both yield and Papaya ringspot virus type W (PRSV-W), fruit quality have been reduced significantly by previously known as watermelon mosaic virus these viruses. Growers constantly ask about the one (WMV 1), and watermelon mosaic virus availability of useable control measures. Inter- two (WMV 2) have been the most commonly estingly, cucumber has not incurred many vi- occurring aphid-transmitted viruses in Florida ral problems for the past three decades or so, for the past several decades. Zucchini yellow except where some exotic varieties were used mosaic virus has occurred in Florida sporadi- in the greenhouse or in the field. cally since the fall of 1981. Cucumber mosaic virus has been less prevalent during the past On a world-wide basis, more than 50 viruses 30 years when compared to earlier periods, but and four viroids have been reported to infect at it has recently increased somewhat in peppers least one member of the cucurbit family. At least and tobacco in north central Florida. Little is half of the viruses are economically important known at this time about the unnamed pathogens of commercial cucurbits. Currently, potyvirus, except that it is distinct from the oth- five aphid-transmitted viruses of cucurbits have ers. Transmission Aphids are efficient vectors of viruses. First, The predominant method of natural trans- they are sometimes able to acquire viruses in mission of the viruses listed in this publication less than eight seconds of probing and trans- in the field is by aphids (sometimes called plant mit them in less than four seconds. Probes of lice, Figure 1) in a non-persistent manner. This 15 to 60 seconds can enhance transmission, but means the viral particles (virions) are acquired longer probes tend to result in lower levels of by the aphid on its stylet (tubular mouthpart transmission. However, aphids may feed for which is inserted into plant tissue for feeding hours at one site if the host is suitable. Second, purposes) and are retained in association with aphids reproduce rapidly, resulting in millions the stylet for “short” periods of time. Although of aphids in a short period of time. Thirdly, alate an aphid is likely to lose its ability to transmit (winged forms, Figure 1) aphids are capable of the virus within a few minutes or a few hours, movement from one location to another either longer periods of retention can occur. For ex- by flying or by passive movement with the ample, two species of aphids, Aphis gossypii and wind. Winged forms tend to develop in re- Myzus persicae, were able to transmit ZYMV at sponse to crowding from other aphids or when levels of 12.5% and 5.6%, respectively, for 3 plants deteriorate. Fourth, aphids can feed on hours at 68° F (21° C) but not at all after 5 hours. a number of plants within a short period of time. In contrast, at 46° F (8° C). A. gossypii could trans- They have “personal” feeding preferences and mit ZYMV at a 1% level for up to 20 hours and will move from plant to plant of the same or M. persicae could transmit at a 1% level for up different plant species until they find a satis- to 30 hours. The actual retention time for sur- factory feeding site. Fifth, numerous species are vival of virions on stylets is likely to vary. capable of vectoring viral diseases. For ex- ample, at least 60, 42, 25, and 12 species of In watermelon, no difference in susceptibil- aphids are known to be capable of vectoring ity to infection to WMV 2 existed in plants from CMV, WMV 2, PRSV-W, and ZYMV, respec- 1 1/2 to 7 1/2 weeks old in an experimental tively, on a world-wide basis. situation. However, symptoms develop at dif- ferent rates depending upon the age of the plant In Florida the number of aphid species at the time of infection. Older plants have an known to vector WMV 2, PRSV-W, and ZYMV, uneven distribution of virus and lower levels are 16, 9, and 13, respectively. Most of the spe- of virus when compared to the younger plants. cies do not reproduce on watermelon, but they However, it is common for plants much older may land and probe while searching for their than 7 1/2 weeks in commercial fields to be- preferred host. It is noteworthy that of these come infected with WMV 2 or other viruses. species only a few species may account for a Low relative humidity (e.g. 40%) has reduced majority of the transmissions in any given situ- transmission of ZYMV by the aphid species A. ation. gossypii but not by the aphid species M. persicae. The retention time of non-persistent viruses is Aphid transmission of viruses is affected by typically further shortened as feeding periods an enormous number of variables, and the in- lengthen. Thus, feeding on non-host tissue af- teractions between variables. For example, the ter acquisition of the virus, and before the sus- efficiency of aphid transmission may or may ceptible crop is reached and probed by the not be altered when the aphid is carrying more aphid, is likely to deplete viral inoculum. Re- than one virus. In one situation, maximum tention time is affected by so many variables aphid transmission of CMV into cantaloupe that it is not advisable to provide generaliza- was delayed by one week when aphids ac- tions for a specific situation. quired a combination of CMV and WMV 2 when compared to CMV alone. Another ex- Primary inoculum of virus comes from weed ample is the extensive variation that exists in hosts, abandoned cucurbit fields, nearby cucur- transmission efficiency between aphid species bit fields, or volunteer cucurbits. Viruses are and clones of aphids within species as they in- introduced into a crop field by flights of aphids teract with other variables. For example, the where often an extremely low number of aphids amount of transmission of WMV 2 from 10 dif- contain transmissible virions. It is generally ferent cucurbit species to cucumber with the believed that because of the “short” retention aphid species Aphis gossypii varied between 8 times of virions on stylets, sources of virus are and 80% in one test. likely to be near the field. However, the reten- tion times of 20-30 hours in some situations The percentage of naturally occurring, mi- suggests that more distant sources of virus gratory, alate (winged) aphids that carry trans- might also be involved. missible virions. has been measured to be as low as 1.5% for ZYMV and WMV 2 in one year. The apparent rapid increase in the incidence In another year, none of 15,122 aphids was of diseased crop plants that we commonly see found to be carrying transmissible virions. In a is the function of secondary spread of virus. The situation with Florida-produced watermelons, percentage of aphids that have acquired virus no aphids with WMV 2 could be detected at from infected crop plants increases as the inci- the earliest time of epidemic development. dence of disease increases. To exemplify how However, when 4% of the plants in a commer- fast epidemics develop, several epidemics in cial field had symptoms, 99% of the aphids in watermelon required only 20 days to progress the species Aphis middletonii were found to carry from zero to 100% incidence. WMV 2. Aphid populations and viral diseases com- Leafminers have been shown to transmit monly progress over time in a sigmoidal fash- WMV-2 and PRSV-W in squash. While this type ion, as do many other diseases. Typically, of transmission can occur, it has not been con- flights of large populations of aphids occur sidered a major method of infection in Florida during dry periods accompanied by moderate in cucurbits. temperature. Such a situation occurs on a regu- lar basis in south Florida during the fall (Octo- Mechanical movement of plant sap from one ber-November) and in south, central and north plant to another is another method by which central Florida during the spring (March-May). these viruses are spread. This can happen any- Within one to two weeks after these aphid time people or equipment move within the field flights, viral symptoms are likely to appear.