Biological Factors that Must be Addressed Prior to Replanting Orchards: Timothy J. Smith, WSU Extension 303 Palouse, Wenatchee, Washington USA 98801 email smithtj@

There has been a great deal of attention paid to the biological aspects of soil when replanting an orchard. There is no other aspect of site preparation that is as important as this one. If your site has built up the "Replant Disease", and almost all have, these living pests will prevent healthy root growth. This disease has great potential to ruin your new orchard, even if you take care of all of the other details of site preparation. Soil pathogens have been controlled on the 1st, third and fifth trees in the picture above.

Not all sites have severe replant disease. Some sites will grow trees well, even without special soil preparation. Some sites grow trees moderately well, which will lead to lesser yields, which is not really good enough in this competative day and age. It is not always obvious that your trees are growing moderately, rather than well. Planting trees on vigorous roots may give you a larger tree on replant disease sites, but the tree will be sick. Small and sick, or big and sick, the poor yield and fruit quality results are the same. Note the apparent poor health of the three year old Gala on seedling root in the picture to the left. The vigorous root did not overcome the replant disease problem. There are years of research, demonstration and local grower experience that shows that 1: replant disease is very common, 2: it is absolutely vital to your economic future that you control it, and 3: it can be controlled.

If you do not understand this subject, seek out all the information you can get before you decide how to proceed.

To quickly summarize the subject:

As your orchard grows, many microorganisms that attack and damage tree roots build up over the years. These fungi, bacteria and nematodes are not always the same in type and number from one orchard to the next. Scientists continue to find new organisms that are involved and proclaim that they have identified the "one" or two that are to blame. This good research show that there is not one cause, there are many pathogens and other growing conditions that lead tgo poor tree growth on old orchard sites. This has made the "cause" very difficult to identify, and control of the pests that causes this disease may be variable. If you plant a young tree into soil that recently supported an orchard (or nursery), these varied pests attack the young tree roots and stunt tree growth. The stunting can be mild to severe. The tree rarely dies, but it performs poorly, and may look nutrient deficient and moisture stressed. Trees suffering from this disease may eventually grow to fill their space, but they never truly recover. Yields will not be as good as they would have been if the disease had been controlled before planting. (See difference replant disease makes 16 years after planting.)

There is no way (yet) to control this disease after planting. Sick trees can be encouraged to grow better by proper care and management, but they will never fully recover. It is vital that you "rest" the soil or treat it before planting to reduce the effect of this disease on the growth of young tree roots.

While much research has been done on replant disease control, and very interesting efforts continue, there are now only two truly effective ways to treat this disease: First, you may rotate your block out of tree production, ripping and fallowing the land for a year, then growing a non-orchard crop for several seasons. Root sprouts allow the pathogenic organisms to survive. A "fallow" orchard with root sprouts growing is almost the same as a growing orchard, as far as the replant disease pathogen complex is concerned. After five to seven seasons without the presence of active tree roots, replant disease will not be a problem. You may be able to reduce the number of years necessary by planting specific crops during this rotation period. Research on these various crop options is under way.

If you have limited land, this long rotation, even if it is reduced by the growing of various rotational ground covers, is not an economical option. The second, and most commonly used control option, is soil fumigation. Rather than wait 4-6 years before replanting, a grower can choose to fumigate and return to full production in that sam 4-6 years. This is the usual more economic choice. In the picture on the right, the 2 year old pear tree in the foreground was planted into fumigated soil, the 5 year old pear tree beside it was planted into untreated soil. Properly applied soil fumigants will effectively reduce the symptoms and effect of replant disease. The effective fumigant choices are presently: methyl bromide, methyl bromide + chloropicrin, Telone C-17 or (C-35) (a mixture of 1,3 DD and chloropicrin), chloropicrin, and the metam sodium or metam potassium products (Vapam, Soil Prep, Busan, Metam, Nemasol, K-Pam, etc.). To date, treatments with alternatives (such as composts) have not proven effective in full-scale orchard trials.

The cost per acre for various soil fumigants and application methods ranges from about $300 to $750 per acre. If your block has replant disease, this money must be spent, but will be returned many times over during the life of the treated block. If you cannot afford to fumigate, you cannot afford to plant.

1986 Methyl Bromide Trial
1987 "Vapam" Rate Trial

Improper soil fumigation is money and opportunity wasted, and may be dangerous to the newly planted tree. Worse yet, poorly applied fumigants may not control the disease.

Replacing soil in the tree planting hole will not effectively treat this disease. Adding fertilizers (such as 11-55-0), composts or fungicides to the planting hole soil does not overcome this disease. Growing various plants such as wheat, marigolds, rape, sorghum, and sudan grass have shown some limited effect, but, to date, tests have been limited and of short duration, and the economics of these treatments are very questionable, (or just plain bad). No silver-bullet soil treatments have been proven truly effective, and all new poorly tested treatments should be considered for test only.

In research trials, the growth effects induced by nontraditional treatments should be compared to the growth of trees properly treated with soil fumigants, as well as to untreated check trees. This is rarely done, as the soil fumigants perform much better than the alternative trreatments. (As an example of this, see the 1986 methly bromide trial above.)

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