Craig H. Canaday and Brad B. Reddick

Two SAR inducers (BioYield and Messenger) and an IPM program that included a third SAR inducer (Actigard) were evaluated for control of common viral diseases affecting summer squash in West Tennessee. The IPM program included soil application of a systemic insecticide at planting (imidacloprid), sprays of Actigard, and roguing of symptomatic plants. Plots treated with SAR inducers showed a gradual increase in the incidence of cucumber mosaic virus (CMV). There was no such increase in control plots. Mechanical transmission of CMV by inducer spray equipment may be responsible for the increase in CMV incidence. The IPM program and the SAR inducers did not significantly affect the onset of flowering, plant height, marketable yields, fruit cull rate due to viral infection, or powdery mildew severity.

Yields of summer squash in Tennessee are often 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 coupled with numerous insect vectors makes 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. Some of the cucurbit viruses can also be easily transmitted mechanically during harvesting or other field operations. Their incidence often dramatically increases shortly after the first fruit are picked.

Several new products that may help control cucurbit viruses are called SAR inducers. These materials induce systemic acquired resistance (SAR) in the host plant. SAR leads to an

activation of biochemical defenses throughout the plant that can reduce disease development and new infections. In last year’s squash test, two SAR inducers, Actigard and BioYield, were associated with significant reductions in the incidence of viral diseases. The latter product is a combination of two species of plant growth-promoting rhizobacteria (PGPR) and is mixed with transplant potting mixes. Both BioYield and Messenger are purported to promote plant growth and fruit yield while at the same time decreasing the incidence and/or severity of plant diseases. This study is an additional evaluation of the efficacy of these products and of an Integrated Pest Management (IPM) program for control of viral diseases of summer squash.

The experiment was conducted in the horticultural research area at the West Tennessee Experiment Station on a Calloway-Henry silt loam complex with high soil test levels of available P and K. A randomized complete block design was used to evaluate six disease-control programs: (1) untreated control, (2) an IPM program, (3) PGPR, (4) the IPM program + PGPR, (5) Messenger, and (6) Messenger + removal of plants with symptoms of viral infection. The IPM program consisted of a soil-drench of imidacloprid + sprays of the SAR inducer Actigard + roguing of plants with symptoms of viral infection before each harvest. Each plot consisted of four rows of Lemondrop L hybrid squash with 12 plants/row. Rows were spaced five feet apart with two feet between plants within rows. To minimize possible drift of SAR sprays between treatments, plots were separated from each other by 15-ft alleys.

Speedling trays (72-cell) were used to produce squash transplants for this study. The trays were prepared June 22 and filled either with Pro-Mix BX or a 40:1 mixture (by volume) of Pro-Mix BX and BioYield, then seeded.

Ammonium nitrate (34-0-0) at 60 lb N/A was broadcast over the test area on June 28. Curbit EC at 4.0 pt/A was applied June 29 as a preemergence herbicide and the test site irrigated (0.5 in. water). Roundup Ultra at 1.0 qt/A was applied July 2 to kill emerged weeds. Squash transplants were set by hand July 3 (reps 1 and 2) and July 5 (reps 3 and 4). For IPM plots, transplant holes received 1.7 fl oz of a mixture of 1.0 fl oz Admire 2F (imidacloprid) in 2.6 gal water - a mix equivalent to 22 fl oz product/A. Plants in all other plots received 1.7 fl oz water.

The test was irrigated six additional times using a lateral boom system that applied 0.4 - 0.6 in. water/irrigation: on July 3, 5, 6, 9, 16, and 23. Sprays of SAR inducers were applied three times each: Actigard 50WG at 0.75 oz/A on July 6, 20, and 31; Messenger at 4.5 oz/A on July 6 and 20 and August 3. All sprays were applied at 20 psi using TXVS-12 hollow-cone spray tips. The first spray was applied when the plants were very small with a backpack sprayer; the remaining sprays were applied with a tractor-mounted sprayer and a horizontal boom.

The number of plants/plot with an open flower was recorded on July 20 to document any differences in the onset of flowering. The height of each plant’s leaf canopy was recorded July 27 to document differences in plant growth. The center two rows of each plot were harvested eight times between July 25 and August 20. Prior to each harvest, all plants in IPM plots and Messenger + roguing plots were examined for any symptoms of viral infection. Plants with obvious symptoms of viral infection were removed. To limit hand transmission of viruses between treatments, IPM plots and Messenger + roguing plots were always harvested before the other treatments.

Leaves from nine plants representing a range of symptoms of viral infection plus leaves from two symptomless plants were collected August 15 and assayed for the presence of CMV, PRSV-W, SqMV, WMV, and ZYMV using standard ELISA techniques. Powdery mildew disease severity ratings were recorded August 20. The percentage of leaf area in the upper ½ and lower ½ of the leaf canopy covered with powdery mildew was recorded for six representative plants in the center two rows of each plot.

Most squash plants survived transplanting. There were no significant differences between treatments in the onset of flowering or in plant height (Table 1). Plants with symptoms of viral infection (e.g., leaf mottling) were first noted on July 20. The proportion of plants with apparent viral infection increased very gradually over the next four weeks - from 2.3% of the plants on July 30 to 4.0% of the plants on August 20. All nine diseased plants assayed for the presence of curcurbit viruses tested positive for CMV infection. No other cucurbit virus was detected. Both symptomless plants tested negative for viral infection.

Increases in the incidence of CMV between July 30 and August 20 were observed only in plots which received sprays of one of the SAR inducers (Table 1). Two factors might account for the observed increases: (1) spread of virus-infected aphids as diseased plants were removed from many of these plots, and (2) mechanical transmission of the virus with the farm equipment used to apply the sprays of Actigard and Messenger. The latter factor seems the more likely one since there was no significant difference in CMV incidence between plots receiving Messenger with or without plant roguing.

There were no significant differences between treatments in the number or weight of marketable fruit (Table 2). Since IPM treatments and the Messenger + roguing treatment required that diseased plants be removed, plots differed in the number of plants being harvested. There were still no significant differences in marketable yields when plot yields were corrected for differences in the number of plants (Table 2). No difference between treatments was observed in the percentage of harvested fruit not marketable due defects caused by viral infection (Table 2).

Powdery mildew was first observed in the test on August 12. The disease quickly spread, particularly in lower portions of the plant canopy where the higher relative humidity favored pathogen sporulation and infection. By August 20, there was a significant difference in powdery mildew severity between the upper leaves in the plant canopy (2% leaf area infected) and the lower leaves in the plant canopy (13% leaf area infected). An occasional plant had >50% of the its lower leaves covered with the powdery mildew pathogen. There were no significant differences in powdery mildew severity due to treatment (Table 3), perhaps because it had been over two weeks since the last spray of an SAR inducer.

Table 1. Effects of an IPM program and SAR inducers on early flowering, plant height, and the incidence of cucumber mosaic virus (CMV) on summer squash, Jackson, TN, 2001.

Email all comments and suggestions to ghonea@utk.edu
Copyright © 1999 by The University of Tennessee. All rights reserved.

This research represents one season's data and does not constitute recommendations.  After sufficient data is collected over the appropriate number of seasons, final recommendations will be made through research and extension publications.