Tree Fruit Leader, Vol. 2(2) Aug. 1993

Report on the International Symposium on Replant Problems

by Gayle Jesperson, P.Ag., Plant Pathologist and
Mike Sanders, P.Ag., Tree Fruit Specialist BCMAFF, Kelowna

The issue of orchard replant problems took on a global perspective at the International Symposium on Replant Problems, held in Penticton from July 21 to 23. Researchers from around the world presented the latest information on replant problems of various crops, concentrating mainly on apple replant problems, but also addressing replant problems associated with stone fruit and even exotic plants such as ginseng and sugarcane. And while the international panel discussed various treatments for this complex problem, fumigation seemed to be the only available treatment that provided benefits over a sustained period of time.

Several speakers mentioned that orchardists in their areas thought fumigation was too expensive, but this point was countered many times by economic analyses showing that fumigation is an investment paying a high rate of return. For instance, Washington state trials showed that each dollar invested in fumigation returned an estimated $160 to $525 over a 5 year period. New York researchers also found that fumigation increased yield and profits of apple for at least 10 to 14 years. Researchers emphasized that the long term effect of fumigation on yield was much greater than the effect on growth. In other words, a tree affected by replant problems may eventually fill its space, but it will never yield as much fruit as a healthy tree. It was suggested many times that orchardists cannot afford to replant without fumigating.

Information from New York researchers also suggested that fumigating for nematodes or replant problem resulted in less winter damage than non-treated plantings. This is a benefit that is hard to put a price on, but is an important consideration.

Cultural factors must also be considered when replanting. For example, weeds can reduce both growth and yields. Research at Summerland has shown that weed control by proper application of herbicides often results in higher yields than weed control by certain non-chemical techniques. However, it was emphasized that the herbicide must be applied correctly. Herbicide damage caused by excessive rates did not always cause any visible injury to the trees, but the yields were reduced nevertheless.

A number of promising treatments for replant disease other than fumigation were also discussed. For example, a bio-control treatment is under development at Summerland. The treatment has shown good results in research trials, including improvements in yield, but more field trials are necessary before commercialization and registration can be considered.

There were also several presentations on the topic of green manure crops. Certain crops may help to reduce replant problems when grown on an orchard site before replanting. Other crops may make the problem worse. This area also needs more research before recommendations can be made.

Unfortunately this symposium did not reveal any magic solutions to the replant problem. Progress on new control methods is always slow from the farmers' perspective. On the positive side, it is encouraging to know that so many people are working on this problem world-wide. By focusing attention on replant problems, researchers hope the symposium will encourage more growers to consider the consequences of not using currently available replant treatments.

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