In replant diseases of apple, two main factors can be distinguished: damage by nematodes, and specific apple replant disease (SARD), not caused by nematodes.
On light soils nematode demage is prevalent, mostly caused by one species, the endo-parasitic Pratylenchus penetrans
(Cobb). This nematode is not very harmful in heavier soils (chapter 2).
On the latter soils, serious problems often occur in replanting apple when no appreciable infestation with plant parasitic nematodes is present, which often is the case. In accordance with this is the observed ineffectiveness of nematicides, and, in particular, of DD, in controlling SARD in these cases.
Because fruit growing in the Netherlands is mostly practised on heavier soils, attention was concentrated on SARD, whose causal factor was unknown. The most important points in characterizing the disease are (section 3.1.):
I . Attack confined to feeder roots, disease not-lethal, recovery of trees starting two to three years after planting
2. Causal factor persistent in the soil for several years, not influenced by any crop so far as known, organic soil amendments have little or no effect
3. Specificity to pome fruit, especially affecting apple
4. Quick recovery of trees and seedlings after being transplanted to fresh soil.
The main point of difference between SARD and the disease caused by Pratylenchus penetrans,
is specificity, which is lacking in the latter case. In the case of SARD there is no obvious cause.
In most fruit growing areas SARD occurred in about 60 % of the soils tested; half of these were seriously infected. In some areas the situation is different: In the Bangert area in the province of North Holland, and in the North East Polder in the former Zuiderzee, the disease occurs generally, while in the South of the province of Limburg the disease is not very serious.
In research on SARD, emphasis has been laid on the practical problems for the grower. Secondly, many observations and experiments were directed towards improving insight into the etiology of the disease. In this part of the work experiments were done on the effect of conditions on disease occurrence, disease transmission and, reproduction of symptoms (section 3.3.). These two aspects of the work were integrated as much as possible in the research programme.
The main result of practical interest (sections 4.3. and 4.4.) is that SARD can effectively be controlled by treatments with a broad spectrum of action against soil organisms, i.e. those treatments which strongly reduce the infestation level of all important categories of soil organisms, e.g. nematodes, fungi, bacteria and actinomycetes. Examples of effective treatments are heat (60°C for 30 minutes) and soil application of various chemicals: chloropicrin, methyl bromide, propylene oxide. In pot experiments thiram was also found effective.
As yet, chloropicrin is the only treatment to be applied on a practical scale. In 4 years experience of this product, it has generally given satisfactory results.
Methyl bromide offers possibilities especially for spring application because it has a lower boiling point and does there fore not demand a long waiting period between fumigation and planting, as in the case of chloropicrin.
Since SARD does not occur in all apple soils, and the treatment with chloropicrin is costly, it is extremely important for the grower to know if a given field is infected. A pot test has been developed (section 4.6.), based on the reproduction of SARD under the conditions of a pot test, and on the correlation between results of CP treatment obtained in the field and in the pot test.
Research on the background of the disease showed that mixing 10% of diseased soil with non-infested soil lead to a marked growth reduction of apple seedlings, but all other attempts to transmit disease or reproduce symptoms failed. These treatments included mixing soils with apple roots and phloridzin, and application of leachates from apple soils. Low pH soils are clearly less heavily infected than near- neutral soils, and acidification of the latter leads to a growth-stimulating effect which is equal to the chloropicrin effect. These observations may offer possibilities for practical application in the future.
When interpreting pH effect in relation to a causal factor different possibilities can be considered: there may be a direct effect on the causal factor, but low pH may also act indirectly, for instance by effecting the nitrogen balance in the soil, i.e. the ratio in which ammonial and nitrate nitrogen are present, which is raised at low pH values by inhibition of nitrification. It may very well be that factors such as nitrogen balance interfere with the etiology of the disease (chapter 5).
The favourable effect of a range of broad-spectrum treatments of the soil in both pot and field experiments, could well be based on the ability of these treatments to control soil bacteria and actinomycetes, because ineffective treatments fail to check these groups of organisms.
In this respect, methyl isothiocyanate and related products are interesting, because they are ineffective in controlling SARD. They control soil fungi and not bacteria and actinomycetes.
In the discussion of the possible causes of SARD, poor soil structure, and chemical deficiencies were ruled out. It seems obvious that soil organisms are an essential factor in the etiology of this disease. Bacteria and actinomycetes deserve special attention because of the selective effect of products as indicated above, the pH effect, and negative indications with regard to fungi. The toxin theory has not been substantiated but it should be stressed that chemicals, including the breakdown products of compounds exuded from the living roots, may very well play a role in the disease process. In such a context, the toxin theory should not be rejected entirely, although toxins are not considered the primary cause.
The need for team work in the continuation of the work has been stressed. It is also clear that more basic knwoledge of the general physiology of the apple tree would be very advantageous to future research on SARD.