Craig H. Canaday and Brad B. Reddick

An IPM approach for controlling the viral diseases affecting summer squash in West Tennessee was evaluated. The IPM program included soil application of a systemic insecticide (imidacloprid) at planting, regular sprays of an SAR inducer, and roguing of symptomatic plants. Use of PGPR was also evaluated, alone and in combination with the IPM program, for effects on plant diseases and yield. The IPM program, PGPR, and IPM + PGPR all significantly reduced the incidence of viral diseases. The percentage of unmarketable fruit due to viral infection was reduced with the IPM program. The severity of powdery mildew was also reduced by treatments that included the IPM program but not by PGPR alone.

Yields of summer squash in Tennessee are threatened by several cucurbit viruses. CMV (cucumber mosaic virus), PRSV-W (papaya ringspot virus type W, formerly called WMV-1), SqMV (squash mosaic virus), WMV (watermelon mosaic virus, formerly called WMV-2), and ZYMV (zucchini yellow mosaic virus) are common in Tennessee. Infected plants produce unmarketable fruit with green stripes, blotches, or mosaic patterns. Plant infection thus limits marketable yields and discourages production.

The enormous number of alternative hosts for these viruses plus numerous insect vectors make control extremely difficult. Many of the vectors are aphids. Some aphids can acquire or transmit the viruses within one minute, making insecticides ineffective for control. Virus problems on summer squash generally increase during the summer months as both the aphid populations and the number of infected alternative hosts in the surrounding environment increase.

One relatively new product for possibly controlling viral diseases is ‘Actigard’ - a material which induces Systemic Activated Resistance (SAR) in the host plant. Another product which has been reported to successfully control viral diseases of some vegetable crops is BioYield 213. The latter product is a combination of two species of Plant Growth-Promoting Rhizobacteria (PGPR) and is mixed with transplant potting mixes. It purportedly promotes plant growth and fruit yield while at the same time decreasing the incidence or severity of viral diseases. This study evaluates combining these products with other Integrated Pest Management (IPM) procedures for control of viral diseases of summer squash.

The experiment was conducted at The University of Tennessee's West Tennessee Experiment Station at Jackson, on a Calloway-Henry silt loam complex with high soil test levels of available P and K. A split-plot design was used with four disease-control programs as main plots: (1) untreated plots, (2) IPM plots, (3) PGPR plots, and (4) plots with both PGPR and the IPM program. The IPM program consisted of a soil-drench of imidacloprid + sprays of an SAR inducer every 7-10 days + weekly roguing of diseased plants. Two yellow straightneck summer

squash varieties, ‘Gold Slice’ and ‘Lemondrop L’, were used as subplots. Each subplot consisted of three rows of squash with 11 plants/row. Rows were five feet apart with two feet between plants within rows.

Speedling trays (72-cell) were used to produce squash transplants for this study. The trays were prepared Jun 15 and filled either with Pro-Mix BX or a 40:1 mixture (by volume) of Pro-Mix BX and BioYield 213. The Speedling trays were then seeded with either Gold Slice or Lemon-

drop L.

Soil was prepared for planting with a disk and field harrow on Jun 23. Curbit EC at 4.0 pt/A was applied Jun 26 as a preemergence herbicide for weed control. Squash transplants were set by hand Jun 26 - Jul 1. For IPM plots, transplant holes received 1.7 fl oz of a mixture of 1.0 fl oz Admire 2F in 2.6 gal water - a mix equivalent to 22 fl oz product/A. Control plots and PGPR alone plots received only water.

The test was irrigated four times using a lateral boom system that applied 0.4 - 0.6 in. water/irrigation: on Jul 3, 7, and 25. Rows were sidedressed with ammonium nitrate at 40 lb N/A on Jul 3 and again on Jul 19. Actigard 50WG at 0.12 lb/A was applied five times to IPM plots (on Jul 3, 11, 20, and 31 and on Aug 7) using a horizontal boom sprayer. All plants in IPM plots were examined weekly for any symptoms of viral infection. Plants with obvious symptoms of viral infection were removed. The center four rows of each plot were harvested eight times Jul 21 - Aug 14. To limit hand transmission of viruses between treatments, IPM plots were always harvested before the untreated plots or plots treated only with PGPR.

Plants with symptoms of viral infection (e.g., leaf mottling) were first noted on Jul 18. The proportion of plants with apparent viral infection increased very gradually over the next five weeks. Ten of the 11 diseased plants assayed for the presence of curcurbit viruses tested positive for viral infection: eight for CMV and two for PRSV-W. Neither of the two symptomless plants from IPM plots tested positive for viral infection.

The incidence of viral infection on Jul 20 and on Aug 4 and 14 was significantly reduced with the IPM program, use of PGPR, or the IPM + PGPR combination compared to the untreated plots (Table 1, bottom). By Aug 23, only PGPR alone or in combination with the IPM program continued to significantly reduce the incidence of viral infection. There was no difference in the incidence of viral infection between the two varieties (Table 1, top). In contrast, powdery mildew was significantly more severe on ‘Lemondrop L’ than on ‘Gold Slice’ (Table 2, top). Only the IPM program, with or without the use of PGPR, reduced the severity of powdery mildew (Table 2, bottom).

The IPM program and the PGPR had no obvious effects on plant growth. The number and weight of squash fruit harvested were also unaffected (Table 3). However, the percentage of unmarketable fruit produced due to the greening, mottling, etc. associated with viral infection was significantly less with the IPM program than in untreated plots (Table 3) .

Table 1. Effects of variety, IPM program, and PGPR on the incidence of cucumber mosaic virus (CMV) and papaya ringspot virus type W (PRSV-W) of summer squash, Jackson, TN, 2000.