This project was designed to investigate the effects of soil solarization and green manure on weed and Verticillium control in a strawberry nursery in the San Luis Valley of Colorado. The weather during the test was unfavorable, with constant afternoon monsoonal clouds and showers, and the tests were terminated 2 weeks prematurely by a catastrophic hailstorm on July 23. The use of green cover crops reduced weed germination and growth. Barley produced the most biomass, but had only intermediate weed control. Mustard had the least biomass, but had low weed growth. Rape produced intermediate biomass, but had good weed suppression. Broccoli produced a good stand but was too thickly sown for optimum biomass production, and had the highest number of weeds. The solarization raised the temperatures of the soil under the tarp appreciably, although the biological effect was unclear. No treatment effects were noted on late planted Chandler strawberry plants or peppermint test plants. Telaromycesflavus was successfully isolated and reapplied to the plots under solarization to test potential biocontrol activity.
1. Determine if biofumigant-green manure species (barley, broccoli, Caliente mustard, Dwarf Essex rape and fallow) with and without solarization influence weed and pathogen (Verticillium) content in strawberry nursery soils.
2.Determine the relative effectiveness of green manure vs. the biocontrol agent, Talaromyces, in controlling weed and Verticillium content of strawberry nursery soils.
3. Determine if biocontrol agents of Talaromyces or Trichoderma, influence weed and pathogen (Verticillum) content in strawberry nursery soils.
Soil from the area was collected from fields previously planted to potatoes, strawberry and barley. After adding lOx the volume sterile reverse osmosis water and stirring for a minute or under constant stirring and used to inoculate Taleromyces isolation
media. Approximately 4000 colonies were obtained within 2 weeks. Of those, 7 had the yellow color indicative of Tale-omyces. Of the 7, 5 turned green within a few weeks, those were discarded as they were not useable. This was expected, and pictures of the cultures were sent to D. Fravel of the USDA for tentative ID. Of the 2 remaining
colonies, the strawberry isolate was subcultured twice on the same media and ultimately on to autoclaved bird food grade millet seed (rehydrated with dextrose water) in 1 liter Mason jars (3/4 filled). After a couple weeks, the millet containing yellow hyphae were
combined in a plastic bag and transported to the field to use as an inoculum. The inoculated seed was spread on the surface of the soil by hand and rototilled in.
Green Manure Cover Crops.
Seed of each species was sown at 30 1bs/acre. The amount used for the barley and mustard and rape were sufficient for a thick stand. The amount used for rape was too much and severe stunting was obtained in the plot center. The amount used for the broccoli was a bit too low and soil surface was easily seen between plants. At the time of
solarization, the following weed counts and biomass dry weights were obtained.
Application of Other Treatments.
Urea was applied as a solid to the surface of the soil. Hydrogen peroxide (as OXIDATE) was applied as a diluted solution to the surface of the soil. Both were rototilled in. The peroxide did make the soil audibly fizz. Trichoderma was added as a drench.
Plots were tarped with 4 mil clear plastic by hand. Ends were sealed by hand by digging a trench under the edge of the plastic and throwing the dirt clod over the outside 5 inches of the sheet. This allowed a tight seal throughout the effective treatment (until the
hailstorm). Maximum wind speed was 12.7 m/sec at 2 m height 2 days after application of the tarp.
The tarp raised the maximum temperature through the soil strata to a depth of 15 inches. The use of clear plastic tarp increased the maximum soil ternperature by 14 C (23F) at 1 inch depth, 5 C (9F) at 5 inches depth, 4 C OF) at 10 inches depth, and 3C (5F) at 15 inches depth. After the hail tore the plastic and allowed heat to leave the ground, the temperature maximum differences were typically around 3 C (5F) at 1 inch depth. Good seal is critical to maximum solarization, as has been previously reported.
Other factors contribute to soil temperature increases under plastic. In our trials, 50C + temperatures were almost always achieved when solar radiation exceeded 300 W/m2. Unfortunately, daily cloud formation and afternoon rains due to a strong monsoonal flow in 2006, prevented effective heat buildup. Wind speed was moderate for most of the experiment, with average velocity ty pically around 2 m/sec. Peak gusts of over 10 m/sec occurred only on three days. Thus, no test conditions were obtained for determining the effect of wind on the effectiveness of solarization.
Diurnal temperature fluctuations in the high elevation desert valley are famously pronounced. Heat built up during the day has an opportunity to dissipate at night. The use of bubble wrap, with its slight insulating and shading properties, decreased maximum temperatures by 2 to 5 C at 1 inch depth, but increased temperatures at 5 and 10 inch depths. There was no effect at 15 inches. The use of green manure mulch, immediately rototilled before tarping, did not add appreciable heat to the soil, although the maximum soil temperatures for the whole experiment were found at one inch depth in the mustard cover treatments: 65.4 C (l49.7F). In that treatment soil temperatures reached 42C at 5 inches and 32 C at 10 inches. Although no chromatographic measurements were taken in 2006, the use of brassicas passed the sniff test. After tillage of the mustard plot and tarping for 2 hours, the plastic was raised to insert the thermocouples. The isothiocyanate odors were reminiscent in type and strength to herbicidal concentration of Vorlex.
The biological effect of these temperatures has only begun to be determined. Weed repopulation has been sparse throughout the experiment. Plug plants of tissue culture; Chandler, Strawberry and Black Mitcham, Peppermint had only occasional and random mortality, with symptoms not associated with Verticillium. Winter survival was
poor and few plants were alive for 2007 measurements.
This project has the potential to reduce chemical application and pesticide use in strawberry nursery production. Because of unfavorable weather conditions during the experimental time periods, clear results could not be obtained. However, observations did
suggest that solarization and green manures have the potential to offer significant weed control in this production system. However, continued research and refinement would be required to make this an accepted practice.
Educational & Outreach Activities
At this time, research findings are not sufficient to justify producer adoption. The findings do suggest a possibility that with further refinement, solarization and biomanures may provide a cost effective method for weed and pathogen control in strawberry nursery soils.
The cold night time temperatures and monsoonal July-August cloud patterns are difficult, but not insurmountable obstacles at improving the performance (if it needs to be) of solarization. Starting the solarization in mid June would mean less cover crop biomass, and subsequently less biofumigant chemicals. The larger amount of biomass produced by rape (vs. mustard), would allow earlier cover crop incorporation. Starting in mid June would also take advartage of higher sun angle and shorter nights. The peak 15 inch soil temperature on the control (untreated plot) was reached in mid July (20th), therefore, a four week solarization period should at least extend one week into June in any case. Cooler initial soil and air temperatures in June would offset only slightly the effectiveness of the tarp.
Can we protect against heat loss during the night? The daily minimum air temperatures fell to less than 10 C (50F) on 21 of the 26 nights of treatment. The use of bubble wrap illustrated the potential for this approach. However, bubble wrap also shaded the soil more than clear plastic. A logical alternative would be to use clear plastic and cover the area with rolled Styrofoam, bubble wrap, woven plastic tarps, or fleece (floating row cover) during the night.
Initial weed control was achieved with mustard and rape and less so with barley and broccoli. Barley is a known producer of allelopathic chemicals, and it achieved the most biomass, so the effectiveness of the brassicas, with lower biomass, was both satisfying and surprising. The effect on weed seed banks was not seen, as germination in the late summer was sparse. Germination will be noted this spring. Fumigation in the fall for spring weed control has been practiced when methyl bromide was widely available. So some overwintering herbicide effect is possible and an expected outcome.