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New Fumigant Trial

Tim Smith, WSU Extension

Second year results of new & old fumigant trial

With the loss of methyl bromide as a soil fumigant coming soon (the year 2000?), it is important that we continue to find effective products and practices for the treatment of replant disease.

Until somebody comes up with something better, fumigation is the best treatment for this problem, and continues to greatly improve the growth of trees in replanted blocks. Most growers who fumigate are very satisfied with the tree growth in subsequent seasons. As with any new practice or material widely used, there have been a very few important problems in fumigated blocks. The most common problem remains the too soon planting of trees into fumigated soil. If we fumigate relatively warm soils in the fall, then plant after the soil has warmed reasonably in the spring, we are very unlikely to see trees damaged. If we rush to plant on cold wet spring appliations, as we did in 1995, some orchards are certain to see fumigation damage.

Since fumigation has been demonstrated as very effective in treatment of replant disease, a few alternative products have been proposed. Some of the products have potential.

In spring 1994, Doyle Fleming set up a fumigant product trial in a block that had been in apples for about 15 years. Various rows were treated with:

1. Metam sodium (the product we have used for some seasons- vapam, soil prep, nemasol, busan),

2. A similar product from Buckman Labs called metam potassium- reported effective at a lower rate than used with metam sodium,

3. Telone c-17- a product that has been used for many years for row-crop fumigation, especially in potato fields (don't confuse this product with telone 2, it's not the same stuff), and,

4. Municipal compost.....Because the trial bordered a block that is being managed organically. Rumor has it that compost alone will overcome the effect of many soil-borne diseases, perhaps it could help in the treatment of orchard replant disease. Perhaps...........Perhaps not........

After the trees (Carousel on M-26) were planted, I measured the trunk caliper of 50 to 60 trees within each treatment. Cross section trunk growth is a good way of measuring vegetative growth of young trees. In the second season, I measured trunk growth. From the 3rd season on, I will measure fruit production only.

Summary of 1995, second leaf results:

The compost treatment, being organic, was weeded by hand 2 or 3 times this season, but the weeds still seem to have the upper hand this fall. Compost was mixed into the soil pre plant, and was piled around the base of trees early in the first growing season. The M26 roots grew into the compost, probably allowing the trees to grow much better than if they had no top dressing. (there are no replant disease pathogens in the compost to slow root growth). Dispite this, the trees in the compost row continue to grow poorly. I suspect this row will have continued problems, even if nursed along very carefully.

There is some slight variation in growth from row to row across the trial, but the general appearance of the remainder of the trial is very good. The soil on this block is very rocky and fairly steep, so the conditions for fumigant application were only fair. Dispite this, the trees in the fumigated rows are growing very well. I expected the telone c-17 would be the most affected by the rocky conditions, as the product must be injected by shanks drawn by a tractor, and large rocks cause the shanks to pop out of the soil. Dispite these application problems, tree growth in the telone c-17 treated rows was excellent.

Results, expressed as percent growth of cross section on the tree trunk ( percent growth is the current trunk cross section area, divided by the starting trunk cross section area):

Telone c-17... 503 a

Metam potassium... 422 b

Metam sodium... 453 b

Compost... 223 c

Statistics indicate that there was no significant difference between the two metam products. There was a significant difference between the telone c-17 treatment and all the others, and the two metam treaments and either the compost or the telone c-17.

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Wenatchee WA, 16 October 1995