This study was carried out for 6 years from 1976 to 1981 to obtain basic data on the control of interns! bark necrosis(IBN) through investigation of various factors affecting the disorder frequently occurring on the dwarf apple orchards in Korea. The results were as follows:
1. The degree of, IBN occurrence and mineral contents in the bark of 1-year-old apple trees on various dwarf rootstocks were significantly affected by pH levels in the nutrient solution. More severe IBN symptoms occurred at pH 4.0 than at pH 6.0. Calcium content in shoot and stem bark at pH 4.0 was appreciably Tower than at pH 6.0, while the contents of N P and Mn were significantly higher. No consistent trend was observed in the contents of K, Mg and Fe in the different dwarf rootstocks according to pH levels. When the same scion-cultivar on different dwarf rootstocks was examined to detect the effect of rootstocks on IBN development, it was revealed that the susceptibility affected by pH levels was low in M9 and M26, while very high in M7, MM106 and MM111.
Z. In the nutrient solution of pH 4.0 and 80 ppm Mn, in which condition IBN could easily be induced, the resistance of the same cultivar on different dwarf rootstocks to the disorder was the highest in M26, approximately intermediate in M9 and the lowest in M7 MM106 and MM111. The relatively higher content of Mn, the primary IBN-inducing factor, in IBN-resistant N26 than in IBN-susceptive dwarf rootstocks suggested that the threshold values of \In in inducing IBN is different in each dwarf rootstocks on which the same scion cultivar was grafted.
3. The degree of IBN symptom in the different parts of 1-year-old apple trees on dwarf rootstocks was recorded. No symptom was observed in the dwarf rootstock part in contrast to severe symptoms in both shoot and stem parts. In the dwarf rootstock part compared with scion parts, the contents of Ca and Fe were significantly high and K was considerably low. Only low tendency was observed in the content of N and P. No consistent tendency was observed in Mg.
4. The scion cultivars of ´Fuji´, ´Megumi´, and ´Starkrimson´ were susceptive to IBN, while ´Jonathan´, ´Orei´, Magnolia Gold´, and ´Spur Golden Delicious´ were resistant. The resistant cultivars have higher Ca content in the leaf than the susceptive ones.
5. The mineral contents in the leaf and bark of various scion cultivars un the same dwarf rootstock revealed that trees with IBN symptom have appreciably higher Mn and lower Ca concentration compared with anormal ones.
6. The changes of Fe content in the bark according to the varying degrees of IBN severity in the same scion cultivar differed from that of Mn and had no consistent relationship among different dwarf rootstocks. Accordingly, the Mn/Fe ratio could not he utilized for the prediction of the degree of IBN occurrence.
7. Under the different regimes of Ca and K where the nutrient solution was conditioned to readily induce IBN with pH 4.0 and 80 ppm of Mn, the higher Ca concentration from 45 ppm up to 130 ppm and the K of 102 ppm than 201 ppm increased Ca content in the bark of 1-year-old apple trees while decreasing K and Mn. Thus, the occurrence of IBN was depressed. This inhibiting effect of IBN development seemed inure closely related with Ca than K.
8. The Ca content was remarkably increased and Mn significantly decreased in the bark of 1-year-old apple trees according to the higher Ca and the lower Mn in the nutrient solution of pH 4.0, resulting in the inhibited incidence of IBN.
9. Under two levels of N in nutrient solution, the high N level (300 ppm) caused more severe IBN development in 1-year-old apple trees and considerably reduced the absorption of Ca and Mg in the bark while increased the concentrations of N and P. Especially, the concentrations of K and Mn increased compared with the standard N level (100 ppm).
10. The summer pruning at the initial development of IBN (June 12), when leaf chlorosis begin to appear, remarkably increased Ca and Mg contents in the barn of I-year-old apple trees while considerably diminishing 1, P, K and Mn contents, resulting in the reduced development of IBN.
11. The foliar spray of 0.5% CaCl₂ had no appreciable influence on IBN occurrence in the apple bark when the Ca concentration in the nutrient solution was the standard level (90 ppm). However, the spray at the 45 ppm of Ca in the solution increased the Ca content and decreased Mn in the bark of 1-year-old apple trees, thus, lead to the decreased IBN occurrence.
12. Conclusively, it was presumed that froth N and K indirectly influence IBN occurrence. Also, it is apparent that the Ca/Mn ratio was much more directly involved in and reasonable for the elucidation of IBN incidence than the simple over-absorption of Mn as the interaction of Ca and Mn, under every- condition: given in this study, were antagonistic.
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