USING NON-WOVEN FLOATING COVERS ON SUMMER SQUASH FOR EXCLUSION OF WHITEFLY - TRANSMITTED GEMINI VIRUSES 1. University of Arizona Tucson, Arizona 85721

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1 USING NON-WOVEN FLOATING COVERS ON SUMMER SQUASH FOR EXCLUSION OF WHITEFLY - TRANSMITTED GEMINI VIRUSES 1 Merle H. Jensen, 2 Mirna Valenzuela 3 and Delmar D. Fangmeier 4 University of Arizona Tucson, Arizona Abstract: The presence of whitefly, Bemisia tabaci, a vector of gemini viruses, makes it impossible to grow summer squash during the fall in the desert southwestern part of the United States. This study was conducted to investigate the effectiveness in covering summer squash with a non-woven floating cover to exclude the whitefly from the squash plants and prevent the transmission of the severe virus. The covers were applied immediately after the squash was transplanted to the field. In just 18 days after transplanting, the floating covers were removed to enable bee pollination. The results were phenomenal with yields increasing 60% if the plants were protected by the covers. Those treatments having plastic mulch along with the floating covers produced yields 160% greater than those plants planted to bare soil without plastic mulch and floating covers. Keywords: Floating covers, drip irrigation, plastic mulch, insect vectors, summer squash. Introduction Growing fall summer squash can be difficult in the desert southwestern part of the United States. This is a time when the sweet potato whitefly, Bemisia tabaci can be at its highest population. This insect is a vector of gemini viruses, which causes squash leaf curl and the physiological disorder, squash silverleaf. At The University of Arizona, scientists were interested in planting summer squash on several types of plastic mulch where drip irrigation was used as a means to water and fertilize the crop. Fertilization of mulched crops through buried drip irrigation is today a well-established technology which was first demonstrated by Jensen in The objective was to evaluate drip irrigation and plastic mulches for growth, quality of the product and the total yield of squash during the fall season, during periods of high temperature. L In partial fulfillment of MS Degree of M. Valenzuela. 2 Plant Sciences Department, University of Arizona. 3 Universidad de Sonora, Hermosillo, Sonora, Mexico. 4 Agricultural and Biosystems Engineering Department, University of Arizona. 159

2 At the time of planting, the sweet potato whitefly was observed in the adjacent cotton crop. As a precaution, the summer squash was covered with floating covers to protect the crop from the insect. A treatment was included where squash was not covered to investigate the effectiveness of the cover in excluding the whitefly. Prior research by Mansour (1991) demonstrated that floating covers offered protection against viruses and feeding damage from insects such as aphids, loopers and beetles. He found that using lightweight fabrics, log/sq.m had a minimal effect on temperature and light transmission. Methods and Materials The summer squash, Cucurbita pepe L. Cvr., "Commander" was seeded August 19 to containers filled with a peat and vermiculite growing medium. The seedlings, grown in a greenhouse, were transplanted to the open field at the third true leaf stage on September 2, The field experiment was established in a complete randomized block design with four replications. The research treatments included two types of plastic mulch, plus a control, and three water regimes. The plastic mulch treatments were: 1. Black/White (white surface up) 1.4 mil polyethylene, plus floating cover. 2. Brown, wavelength selective 1.0 mil polyethylene, plus floating cover. (This mulch absorbs most of the photosynthetic active radiation (PAR), providing weed control, since the PAR is blocked, but transmits a large portion of the near infrared radiation (NIR) for soil heating.) 3. No polyethylene mulch with floating cover. 4. Bare soil - No polyethylene mulch without floating cover. The three irrigation levels were: 1. Dry (tensiometer reading: 0.55 bars at irrigation). 2. Medium (tensiometer reading: 0.40 bars) and 3. Wet (tensiometer reading: 0.25 bars). The tensiometers were installed 30 cms deep into the soil beds. The plots were irrigated with Chapin Turbulent Twin Wall IV drip lines (rated at 0.5 gal/min/100 ft at 10 psi). The lines were buried at the center of the growing beds 15 cm below the soil surface. The plastic mulch treatments were installed after the drip lines were buried. The floating covers were installed immediately after the plants were transplanted to the field plots, in order to reduce the potential risk of virus infections from insect vectors and damage from other insects (Fig. 1). The fabric weight of the cover was log/sq.m. The squash plants were spaced 50 cm apart in the row with 24 plants per replication. Each row was 1.0 m in width. A blank row was left between the plots to prevent any interaction among the three irrigation treatments. The plants were protected with the covers for eighteen days. The covers were removed from all the treatments on September 20, after the plants began to blossom, in order to allow bee pollination to occur. To observe early growth, the number of female and male flowers were counted from each plot at the time when the covers were removed. Height of plant data was also collected at this time. 160

3 Fig 1. In order to exclude the whitefly and other insects, the non-woven floating covers were installed immediately after the squash plants were transplanted to the field. Yield data was collected from 10 plants of each replication starting September 23. The weight and length of each fruit were recorded. The fruit was evaluated on the basis of texture, color and incidence of disease. Results and Discussion While this paper is primarily focused on the effectiveness of non-woven covers in protecting the plants from insects, observations on the plant response to plastic mulches and irrigation levels are included since all of the above factors are related in regard to crop response. Water Level Effects. The difference in the total amount of water given to the plants at the three irrigation levels was not as large as expected. The average volume of water for the mulch treatments was 14,812 liters for the dry plots, 14,213 liters for medium irrigation and 16,906 liters for the wet irrigation plots. The average water use between mulch treatments seemed to have a direct relationship to the total growth of the squash plants. The black/white mulch treatment used an average 17,661 liters versus 14,707 liters for the bare soil treatment. While there were no significant differences for plant height, number of flowers and total season yield between irrigation levels within each mulch treatment, there were significant differences between the mulch treatments, for the mean height of the squash plants and the yield. The plant height of those plants growing under the floating covers, whether they were the mulched plants or on bare ground, was about twice that of plants growing on bare ground without the floating cover (Fig. 2). 161

4 Fig 2. Plants not covered with floating covers were greatly stunted by squash silverleaf and leaf curl virus. On September 21, the day after the covers were removed, the plants that were protected by the floating covers ranged in height from cm while those plants having no cover had a height of 24 cm. This height difference prevailed to a degree during the entirety of the project. Weed Growth Effect and Insects. At the time the floating covers were removed, there was no weed growth under the black/white plastic mulch or the brown, wavelength selective mulch. However, there was heavy weed growth around the squash plants growing under the floating cover with no plastic mulch. The weeds caused an elongation effect on the squash stems, mostly due to the competition for light. The weeds were removed, as well as those weeds growing around the squash plants on bare soil with no floating cover. On September 5, the first insects (aphids, whiteflies and cucumber beetles) were observed on those plants planted to the bare soil treatment with no floating cover. On September 12, the first symptoms of squash silverleaf were found and later stunted growth occurred. Once the floating covers were removed, the first symptoms of squash silverleaf and squash leaf curl were observed on October 4 on the brown mulch plants. This could have decreased the quantity and quality of the squash fruit more in the brown mulch treatment than the black/white treatment. Yield Highly significant differences in yield were observed among the mulch treatments (Fig. 3). On October 8, mulched treatments, black/white (BAV) and brown (BR) did not show significant differences during the mid-season harvest for total and market yields, but they were significantly greater compared with the no mulch with cover (COY) and the bare soil, no floating cover (BS) 162

5 treatment. However, on September 30, at the end of the season, highly significant differences were found among all the treatments for total and marketable yields. BAV showed the highest total and market yield. B J CL o e 3 0) Bl/White BROWN COVER BARE SOIL Plastic treatments Mid-S T H Mid-S Mkt ^ Final T. Final Mkt Fig 3. Mid and end of season total and marketable yield comparison among black/white and brown mulch, no mulch with floating cover and bare soil with no cover. This most likely occurred due to the lower soil fall temperature under the BAV mulch, which had the white surface on top versus the brown mulch, which had warmer soil temperatures. It is believed that the yield from the COV treatment was lower than the two mulch treatments because of the early weed growth around the COV plants. The low yields from those plants in the BS treatment, was undoubtedly due to the curl leaf virus and the squash silverleaf. It didn't appear that the high air temperatures of early fall, over 35 C, harmed the small squash plants under the floating covers. While the brown wavelength selective film would probably be ideal for use in the spring, due to the transmission of heat radiation, it is not advisable for use in late summer or early fall in a desert region. If used in the spring and again in the fall for a second crop, it may be advisable to spray the brown mulch with a white latex paint. 163

6 While this research should be repeated to establish which mulch is best for use in the fall season, there is no question about the advisability in using floating covers to protect young plants from the invasion of the sweet potato whitefly and other insects. Conclusions The results of this study show that the three water levels applied to each plastic treatment did not significantly affect the plant height, the number of flowers, or yield. However, the results did show that the plastic treatments significantly affected the plant height, the number of flowers and yield of squash. Black/white mulch with the cover achieved the highest yields as compared with the brown mulch with cover, the cover alone with no mulch, and the bare soil with no cover. It is presumed that black/white plastic mulch had cooler soil temperatures with the white surface probably improving the reflective radiation balance around the plant at early growth stages. The eighteen days of protection with the cover over the mulched plants were sufficient to protect the plants against insect virus vectors, such as whiteflies. As a result, black/white with the cover, achieved 92.9% of the U.S. average yield. Weeds were controlled with the use of black/white and brown mulches, but not with the use of the cover without the mulch or with the bare soil treatment. The black/white and brown treatments showed the highest water use efficiency; that is, greater yields per volume of water compared with the cover/no mulch and bare soil treatments. For late summer and early fall, the combination of plastic mulches with floating covers and drip irrigation is a high-cost technology and should be evaluated for other high-value crops such as cantaloupes, watermelons, etc. This technology offers new opportunities for growing crops during periods of the year when insect vectors are at their highest population, often at a time when market prices are their best. Literature Cited Jensen, M. H. and R. Sheldrake, Jr Concluding results of air supported row covers for early vegetable production. Proc. of National Agricultural Plastics Conf. 6: Mansour, N. S The use of field covers in vegetable production. Proc. Intn. Workshop on Imp. Veg. Prod. Through the Use of Fert., Mulching and Irrigation, Chiang Mai Univ. Thailand. 164