The terms alternate, biennial, intermittent or irregular bearing for mangoes are used to describe the tendency of mango trees to bear heavy crops in one year and very little or no crop at all in the next year. These terms should not be confused with shy bearing or unfruitfulness as trees are said to be shy bearers or unfruitful, when under acceptable conditions trees produce a few fruits or no crop at all. This condition arise if the variety is inappropriate to the particular climate which is not favorable to the appropriate vegetative- reproductive balance for that variety to bear a profitable crop.
Trees or varieties having a biennial bearing habit bear a heavy crop in one year which is termed ‘on year’ while in the next year they have very little or no crop which is termed ‘off year’. It is a proven fact that this tendency of on and off remains so and goes continuously unless the habit is altered by external factors such as diseases or pests, inclement weather at flowering or by growers deliberate attempts by proper tree management.
The management of biennial bearing become important as the profitability of the orchard depend on the production of reasonable crop every year. Therefore an attempt is made here to analyze the problem for growers to get an understanding before going i nto manage it.
The biennial bearing may be caused by one of the following factors or in combination of one or more factors.
4. Tree management
Varieties with a long growth cycle show natural biennial tendency. Mangos show clearly demarcated and very distinct growth patterns during an annual growth cycle. New growth flushes emerge after harvesting and these flushes mature and remain dormant for some period before the trees flower. Again from flowering to fruit harvesting there is a period called fruit development. After harvesting after a short time period again the vegetative flushes emerge and this growth cycle continue.
Time spans for the periods from harvesting to flushing, from flushing to flowering and from flowering to harvesting vary according to variety. Time taken from flowering to harvesting may vary from 3.5 to 5 months for a range of mango varieties, and based on these the varieties may be classified as early, mid season and late varieties. Usually for a single variety time taken from flowering to fruit maturity is fairly constant and may slightly vary with climatic conditions experienced during fruit deve lopment.
Thus, for late varieties it takes more than 12 months to complete a growth cycle. Then after giving a heavy crop in one year, late varieties have less time period before next flowering to produce the flush and to accumulate reserves for the next crop. Then that crop in the second year may be poor or some times trees may not flower at all if the shoots are not physiologically conditioned for flowering by the time next flowering season starts. However, by the third year, again the tree will have accumu lated more reserves to support a heavy crop. In this way, biennial production pattern continue in such varieties.
It is therefore very difficult to completely overcome this problem if it is originated as a result of varietal character. Nevertheless, this problem may be minimized to a certain extent by controlled application of N and water at least 2 weeks before harvesting fruits to induce an early growth flush. This technique is used to reduce the time period taken for a growth cycle as much as close to 12 months.
Even if a regular variety is grown under rainfed conditions, irregular bearing may be observed, not necessarily biennial bearing, in line with the variations in climatic conditions. The primary climatic factor affecting the irregular bearing is rain f all. If rains delay after harvesting, there will be a longer time period between harvesting and the following growth flush. Then the time period available for trees to have the rest before flowering and accumulate food reserves is lower than that happen under a normal season. Under such conditions, poor crops will be produced in the following season.
If mango orchards under rain fed conditions face this kind of irregular bearing, then the only option available to control it is to provide irrigation facilities. In doing so the productivity of the orchards may be significantly improved.
If mango orchards have been established in very fertile, deep rich soils or in places with high water tables, or in locations with lot of shade etc. irregular bearing may be a problem. Under such conditions, it is difficult to control the vegetative growth of trees. Especially in places where growth checks can not be achieved between flushing and flowering as a result of too much N or too much water, irregular crops may be produced. Under this kind of situation also the crop load depend on rainfall. However, heavy crops may be possible only when drought periods occur, because it will affect the availability of N and water to trees which is required for growth control for succesful flowering and fruiting under such conditions.
To control irregular bearing under those circumstances, chemical growth retardants may be quite helpful. Also change of varieties is another possibility. Under dry or intermediate zone conditions, mango variety Vellaicollomban may be more suitable for locations with such conditions than Willard or Karthacollomban as the latter varieties need well defined dormant period with less water as a pre condition for good flowering
If the variety, growing environment and planting site is alright, still the biennial or irregular bearing problem might exist due to lack of a proper tree management. If the trees are subjected to a management system based on tree phenological cycle as described in earlier chapters, this irregular bearing may be controlled effectively. The most important management tools available for control of irregular bearing is N and irrigation. Therefore, special care should be taken to apply N and water at right time as described previously.
In addition to the above two inputs, crop protection also affect irregular bearing. If flowers are not protected from pests like mango hoppers or from diseases like anthracnose, the crop will be very poor in that season. However, the following season crop may be little higher due to the buildup of more tree reserves esential from flowering to harvesting to control this.