Yield and Economics of Intervention with Peach Tree Short Life Disease

Abstract

Fumigation, rootstocks, and time of pruning of peaches [ Pru‐nus persica (L.) Batsch] were studied to determine if the cost of soil fumigation would be recovered when replanting peach trees on a peach tree short life (PTSL) site. Incidence of PTSL, yield, and economic return were determined over 3 yr. Treatments were fumigation with 98% methyl bromide or no fumigation, ‘Lovell’ or ‘Nemaguard’ rootstocks, and pruning in December or March. Pruning effects were inconsistent from year to year and did not affect the 3‐yr‐cumulative yield. Fumigation increased yield per tree and decreased tree loss due to PTSL. Significant interactions between fumigation and rootstocks indicated that fumigation increased yield per tree more with Lovell than with Nemaguard rootstocks. Increased yield and decreased mortality following fumigation increased economic return by 31% over the nonfumigated treatment. Soil fumigation increased yields sufficiently on a PTSL site to justify the practice economically. Research Question Peach tree short life (PTSL) is a serious replant problem affecting peach production and tree longevity in the Southeast. Soil fumigation with methylbromide can increase trunk circumference and tree size of peaches and decrease ring nematode (Cx) populations and tree loss due to PTSL. Soil fumigation, however, is expensive, which often prevents growers from fumigating. The purpose of this study was to determine whether the expense of fumigation could be recovered by increased production and tree survival in a severe PTSL orchard. Literature Summary The actual causes of tree death in the PTSL complex is cold injury or bacterial canker, but trees are predisposed to PTSL by Cx, fall pruning, improper rootstock selection, low soil pH, and planting on land previously used for peach production. In the major peach production areas in Georgia, land which has never been planted with peaches is scarce and growers often must replant peach trees in “old” peach soil, which greatly increases the incidence of PTSL. Preplant soil fumigation to control Cx populations has been recommended for planting peach trees in old peach soil. Soil fumigation has been shown to improve growth of apple, cherry, and pear trees in replant situations as well as increase tree longevity in PTSL sites. Study Description The experimental site was on an Orangeburg sandy loam soil in the major peach producing area in central Georgia. The site had at least three previous plantings of peach trees prior to this experiment. Tree longevity decreased with each planting from about 20 yr with the first planting to only 4 yr for the third planting. About 40% of the 4‐yr‐old trees had died of PTSL before orchard removal. Fertilizer and pesticide applications followed University of Georgia Extension Service recommendations. Tree spacing was 15 ft within rows and 20 ft between rows. There were nine trees per treatment plot and six replications for a total of 864 trees. Virus‐free rootstock and ‘Red‐haven’ scions were obtained through the South Carolinaa Department of Agriculture certification program. Lime treatments had no effect on tree survival or growth and were not included in yield determinations. All fruit were hand harvested by commercial pickers at the threshold mature stage at each of three harvest dates. Yield was calculated from the number of 38‐lb box units of fruit per acre for each fruit size grade. Cash value for each grade on each harvest date was determined from daily f.o.b. prices at central Georgia shipping points. To determine final cash return, it was assumed that 15% of the yield from each tree was cull fruit and could not be sold, and it cost $2.25 to pick and $1 to pack each box of fruit. Treatments: Limed or not limed Fumigated with methyl bromide (500 lb/acre) or not fumigated Pruned in December or March ‘Lovell’ or ‘Nemaguard’ rootstock Applied Questions Did fumigation, rootstocks, and time of pruning affect yield per tree or incidence of PTSL? Only funigation consistently increased yield per tree in all three years (Table ). Yields of fumigated trees were more than 20% greater than nonfumigated trees in all 3 yr. Time of pruning influenced yields in 1988 and 1989, but the effects were inconsistent from year to year and did not affect the 3‐yr cumulative yield. Fumigation had a greater affect on trees on Lovell than Nemaguard rootstock. The differences between rootstocks probably were associated with rootknot nematode resistance. Rootknot nematode population densities ranged from 29 to 175 nematodes/39 cu in. soil under Lovell trees and from 0 to 12 nematodes/39 cu in. soil under Nemaguard trees. Did the increase in yield and tree longevity make fumigation with methyl bromide an economically practical management option? In each year of the study, higher tree loss and lower yields reduced dollar return in nonfumigated treatments (Table ). At the end of three growing seasons, the gross accumulated return per acre was $1379 higher with fumigation than without fumigation. Assuming a $1000/acre fumigation cost, a peach grower could increase gross returns by an additional $379/acre if planting in a PTSL site. Although yields were not determined in 1990 and 1991, the return from investing in fumigation probably would increase because fumigation decreased the incidence of PTSL and thus more trees survived to harvest. In 1990, the percentage of dead trees due to PTSL was about 6 and 39% in fumigated and nonfumigated treatments, respectively. Similar results were obtained in 1991 with twice as many trees dying in nonfumigated treatments than with fumigation. Recommendation The dual effects of fumigation on yield per tree and incidence of PTSL increased yield per acre and subsequent return by 36% over the nonfumigated treatment. The advantage of fumigation in terms of gross return per acre should be taken into account by growers facing a replanting decision. Effect of fumigation and rootstocks on peach yields per tree and percent PTSL. Year 1987 1988 1989 Lov Ngrd PTSL Lov Ngrd PTSL Lov Ngrd PTSL ‐‐‐lb/tree‐‐‐ % ‐‐‐lb/tree‐‐‐ % ‐‐‐lb/tree‐‐‐ % FUM‡ 56.5 53.5 0.23 55.5 62.6 0.47 88.9 80.1 1.63 NF 38.9 49.2 6.74 48.4 50.6 24.24 65.6 75.6 29.84 Effect of fumigation and incidence of PTSL on actual yield of peaches and gross return per acre. Gross return based on daily f.o.b. prices at central Georgia shipping points. Yield Return Year FUM NF Cumulative difference FUM NF Cumulative difference ‐‐‐‐‐‐‐‐‐‐lb/acre‐‐‐‐‐‐‐‐‐‐ ‐‐‐‐‐‐‐‐‐‐$/acri‐‐‐‐‐‐‐‐‐‐ 1987 7 960 5 950 2 010 $448 $400 $88 1988 8 510 5 320 3 190 $1142 $715 $427 1989 12 200 7 330 4 870 $2167 $1303 $864 Total 28 670 18 600 10 070 $3797 $2418 $1379 Lov = Lovell rootstock; Ngrd =

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