In recent years, the bark splitting injury has occurred frequently both on cuttings in nursery and on young plants in tea gardens at southern districts of Kyushu.
In order to clarify the characteristics of the bark splitting injury, the various field conditions and the climatic factors easy to cause it were investigated.
1. The bark splitting injury occurred on the basal part of stem of both cuttings and young plants up to two-year-old was chiefly observed at the first frost near the ceasing time of the autumn growth.
The occurring mechanism of this injury is expressed as that the bark of stem near the ground surface splits open lengthwise owing to ice formed between sapwood and bark under the continuous freezing by the effective low temperature (about-3 to-5°C). The bark split recovers with growth of the callus in case of the slight damage but if it is severe, the plant weakens gradually and will finally die.
2. From the observations of recovering process after the injury, the symptoms of the occurrence of the injury were classified in three different types : a) the bark of stem near the ground surface splits lengthwise, b) the bark separetes from sapwood without the bark split and c) the cambial tissue becomes brownish without the bark split.
In the case of b) and c), the injury remains as a frost-ring on the stem.
3. Both the cold injury during midwinter and the bark splitting injury in late autumn or early winter cause the young plants to be seriously damaged. The reasons for the above are as follows : 1) the root systems of young plant are poor, 2) the stems of young plant are scarcely protected from low temperature by their own leaf layer and 3) the acquiring time of the herdiness of young plants is late because they grow actively till late autumn.
4. The minimum air temperature at about five centimeter above the straw mulch on fine days in Chiran, where the injury occurred frequently and severely, was 6 °C lower than that of Makurazaki, where no injury occurred, while the mean and maximum air temperature of both sites were almost same.
So the primary climatic factor responsible for the occurrence of the bark splitting injury in Chiran is assured to be sudden down of the minimum air temperature at the first frost in autumn.
5. The minimum air temperature in the field mulched with the straw was lower than that of non-mulched field, and the mulching kept the soil temperature high in night. These effects were remarkably great on fine day. Therefore, it was considered that the straw mulch promoted the bark splitting injury because of the high soil moisture, the high soil temperature and the low air temperature near the mulch surface at the frosty night.
And then, it was also pointed out that the average of the minimum temperature for ten days before the first autumn frost was higher in the injury years and places than in the normal ones.

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The influences of air temperature, soil conditions and fertilization on cold resistance or on the bark splitting injury of young tea plants were investigated using artificial bark splitting technique, electric conductivity method and browning test.
1. The bark splitting injury was not affected by the diurnal range but by the minimum air temperature in the time of the occurrence.
2. The soil temperatures did not affect the occurrence rate of the injury but the size of the split caused under the higher soil temperature was larger than that caused under the lower soil temperature and gradually enlarged with the time of the freezing treatment.
3. The occurrence of the injury markedly decreased with the reduction of soil moisture.
4. The hardening of the young tea plants was accelerated by 3 day's exposure to the air temperature of 0°C.
The increase of the cold resistance by the low air temperature above mentioned was observed 1 week later slightly, and remarkably observed 2-3 weeks later.
The young tea plants exposed to such low air temperature in middle time of the hardening obtained approximately equal cold resistance regardless of the amount of nitrogen applied in autumn.
5. The tea plants in hardening was not dehardened even by the air temperature of 25°C for 1 week.
6. The hardening was gradual untill midOctober or early November but became rapid from about mid-November.
This process almost coincided with the increasing process of the resistance to the bark splitting injury.

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1. The bark splitting injury has frequently occurred in various parts of warm region in recent years. A wide range of resistance to the bark splitting injury under the field conditions also has been observed among the tea clones.
The resistance of the clonal varieties was ranked by means of both the observations of the injury at the various sites of Kyushu district and the results obtained by the newly developed testing methods.
The ranking of the hardiness among the main clones was shown as follows : Kanayamidori> Yabuki to>Asatsuyu>Sayamamidori>Okumusashi, Yaeho (hardy>tender).
2. It was cleared that the resistance to the injury did not correlate with the frost hardiness of mature leaves in mid-winter which closely influences northern limits of commercial cultivation of tea plants, the time of bud opening, the vigor of plants and the quality of green tea.
3. The following three testing methods for the bark splitting injury were proved to be effective : 1) testing for the frost hardiness of the basal stem of young plants at the period of the first autumn frost, 2) testing for the easiness of artificial occurrence of the bark splitting induced by the repeated freezing treatment and 3) testing for both the frost hardiness and the cambial activity of the mature shoot of adult plants.
The cambial activity was estimated by easiness of peeling the stem bark from the sapwood and by water content of stem.
The last method is simple and easy to be applied to the actual breeding procedure.
4. The best period for the testing of resistance to the bark splitting injury is confirmed to be late October to the end of November at Tea Branch of Miyazaki Agricultural Experimeut Station.

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The influence of manuring methods on the cold resistance of young tea plants during the occurrence period of the bark splitting injury was investigated.
The young tea plants fertilized with one third of the current doses of nitrogenous fertilizer and with no nitrogenous fertilizer in autumn, (Sept. to early Oct.), obtained the cold hardiness more early than the normally fertilized one did.
However, normal usage of autumn fertilizer in early Aug. after summer fertilization was omitted and the normal doses of nitrogen if their application was made very early, such as in July or suspended untill such late time as Nov. accelerated the cessation of autumn growth and the hardening of the young tea plants.
The growth of the young tea plants well-fed with basal manure plus summer dressing was not affected by the time of autumn manuring.
The potassium effect on cold resistance of the tea plants was not detected, although there has been many reports on it.
These results suggest that the later manuring in autumn is better choice for prevention or reduction of bark splitting injury in dangerous regions.

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Covering methods with some kinds of grasses and chemical textures were studied from 1973 to 1975 for the effectiveness in preventing the bark splitting injury that is a kind of cold injury caused by frost and is mostly found in young tea plants grown in warm regions.
1. It was proved that all the covering methods protected young tea plants from being exposed to extremely low temperature in occurrence time and consequently reduced the damage of the bark splitting injury.
2. The type of grasses that are tolerable to a low temperature, such as oats, showed greater effect than sorghum or crotalaria which is tender to a low temperature.
3. The covering method with oats showed great effect in the case that they had perfectly covered the young tea plants, when the amount of insolation was 40-60cal/cm², before the occurrence season of the bark splitting injury.
The best result was obtainded in the case that the seeds fo about 4kg/10a had been sowed in belt on both sides about 30cm apart from a row of the tea plants in late August.
4. Straw mat and chemical textures (BB-tex, Tuffubel and cheesecloth) used as covering materials were also effective to prevent the injury but the covering effect of the chemical textures was smaller than that of the straw mat.
5. The covering method with oats showed greater effect than any other methods probably because the method kept low the maximum air temperature under tea plants' crown and controlled the supply of moisture and nitrogen in soil to tea plants through competition.

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The ridging to the basal stems of young tea plants, which was applied to 15-16cm above the ground before or iust after the occurrence of the injury, was examined for the effectiveness in reducing the bark splitting injury which was a kind of cold injury by autumn frost and occured mainly on stems.
1. The previous ridging did not maintain high the minimum air temperature near the soil surface at the occurrence time of the injury.
2. The previous ridging did not also restrain the occurrence of the injury but the tea plants damaged after ridging showed good growth and high rate of survival later, probably because the most important basal stem was protected from the injury by ridging.
3. The ridging just after the injury remarkably accelerated callus formation at wounded parts and consequently reduced the damage.
4. From the above findings, the forehand ridging for prevention is advisably practiced in the regions where severe injury is anticipated, while the post-injury ridging is also a valid method for remedy.

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