Limited Sowing Time and Lack of Moisture at Sowing
A mung bean crop grown in summer must be harvested before the onset of the monsoon to avoid early rains. Therefore, if the last date of harvest is assumed to be 20 June and if the crop duration is about 70 days then the last date of sowing would be 10 April. A date-of sowing experiment showed that sowing in late March gives better results. Traditionally farmers grow pulses under rainfed conditions and if they have irrigation facilities they will not grow mung beans. The probability of receiving 10 mm rainfall in the second week of April ranges from 40-60% in the northern districts, based on which no production plan could be successful (Manalo 1978). Sufficient rain for sowing occurs at the end of April or early May. If mung bean is sown at that time the crop will mature in July making harvest difficult. The farmers will then have no choice but to plough down the crop ( BARI 1984; OFRD 1987).
Moisture Variation at Planting and Maturity: Climatic factors primarily responsible for the adaptation of mung bean and black gram (Vigna mungo (L.) Hepper) are day length, temperature, humidity, and rainfall. Although mung bean and black gram are reported to be drought-tolerant crops (Morton et al. 1982; Rachie and Roberts 1974), they are susceptible to moisture stress during the establishment period and flowering stage (Rosario and Faustino 1985) The optimum temperature for mung bean and black gram ranges from 28-30°C (Lawn and Ahn 1985). It may be possible to sow these crops in Bangladesh during March in some years but they may suffer from prolonged drought during April/May, due to high temperature (34-36°C), low humidity (46-60%), high potential evapotranspiration rate (5-6 mm day-1), and a negative water balance (139-165 mm). The aridity index is reported to range from 0.11 at Dinajpur to 0.24 at Pabna (Manalo 1978). These factors result in poor plant growth, wilting and abscission of flowers, leading ultimately to poor yields. If excess rains occur during this period, wet and humid conditions stimulate vining, luxuriant growth, and favor incidence and spread of foliar diseases (Lawn and Ahn 1985), again resulting in low yields.
Problems During Harvest: Farmers of the targeted area are not accustomed to hand picking as a means of harvesting the pods. But this practice cannot be avoided during the summer. Rains generally occur by the end of M a y when the crop is at the pod-filling stage, thus increasing soil moisture and humidity. Long days coupled with favorable soil moisture stimulate successive flushes of flowers. Thus individual racemes may simultaneously bear flower buds, flowers, green, and ripe pods. In black gram, the growth of new leaves and fruiting continues and the ripening period extends over many weeks. It has been found that only about 65% of pods can be harvested in the first picking at 72 days, 18% in the second at 82 days and 17% in the third at 97 days. So at least 90- 95 days are required for a complete harvest. If it rains continuously over 2-3 days during the maturity period, seeds in mature pods will sprout or may get discolored and shriveled, lose viability, and spoil or become infested with fungi. Thus harvesting, drying, and threshing become more difficult.
Biotic Stress Factors: The yield potential of mung bean is not realized because of disease and pest damage. So far, eight diseases have been identified, of which yellow mosaic virus (YMV), Cercospora leaf spot (CLS), and powdery mildew (PM) are the most serious (Ahmed et al. 1982; Fakir 1983). Sources of resistance have been identified for all the major diseases except for YMV, for which only moderate resistance is available (AVRDC1983; Sandhu 1978).
Abiotic Stresses: A mong the abiotic stresses, soil type and fertility play a major role. Both mung bean and black gram perform best on deep, well-drained loams and sandy loams (Mehta 1970; Rachie and Roberts 1974) and neither are well suited to the shallow or unfertile sandy soils on which they are often grown. It was reported that a crop-producing 1t yield removes 40-45 kg N, 3-5 kg P2O5, 12-14kg K, 1.5-2kg each of Sand Mg, and 1-1.5 kg Ca from the soil (Lawn and Ahn 1985). However, farmers do not use any fertilizer or perform intercultural operations.