I have been working with World Vision on rehabilitating cashew for the past four years. Because cashew has been mentioned a number times in EDN, I thought I would write with more information.
Cashew (Anacardium occidentale) may be known as a poor man’s crop, but this is probably because it grows well on sandy soils of poor fertility. In Mozambique and Tanzania, cashew occupies lowland areas close to the coast with a rainfall between 900-1300 mm (35.5-52 in) per year. Cool temperatures of less than 7°C (45°F) kill the flowers. That is why cashew does not thrive at altitudes higher than 500 m (1640 ft) in Mozambique.
Processing of cashew is both complicated and costly because of the existence of CNSL (cashew nut shell liquid). This liquid contains 90% anacardic acid and 10% cardol. These liquids are very caustic and when heated the fumes are poisonous. Related compounds can be found in poison ivy. In fact, people sensitive to poison ivy are often found to develop skin sensitivity to CNSL. Local processing consists of heating the raw nuts over a flame long enough to burn off the CNSL, which is transformed to a thick toxic smoke. The kernels are of low quality and fit only for local consumption. Except for enough trees to meet local demand, I would not encourage the planting of cashew in areas where there is no processing industry. However the value of cashew increases considerably where there is a cashew processing industry which exports the kernels to Europe or the U.S. World demand for cashew is increasing and the future for increasing nut prices looks bright.
In East Africa yields of cashew have been devastated by powdery mildew (Oidium sp.), which kills the flowers. Without treatment yields are very low, 0-5 kg (0-1.1 lb.) per tree. With fungicide treatment yields can be 15-40 kg (33-88 lbs.) per tree. Some farmers treat by dusting with sulfur but I can’t recommend this due to the acidifying effects of sulfur on sandy soils (which are already acid).
We are trying to improve cashew yields without chemicals. This is done by pruning suckers on lower branches. These tend to be highly infected by powdery mildew and a source of spores for future infection. Weeding is also encouraged to eliminate competition at the base of the tree, make harvesting easier, and as a firebreak. In Mozambique uncontrolled burning is the major killer of productive cashew trees. Helopeltis anacardii, a sap sucking insect that can cause much flower damage, is the second major pest.
A statistician has called cashew the most variable agricultural plant he has worked with. We have trees that have produced 40 kg of nuts one year decrease to zero production the following year (largely because of disease). Rainfall, insect infestation, humidity, and temperature can all affect yields in a variety of ways. This variability makes research difficult and lessens the value of cashew to poor farmers who need regular income. Those who seem to make money off of cashew tend to be the farmers and commercial operators who can implement a regular fungicide spraying program.
As far as labor is concerned, much of the labor needed is at harvest (which comes during the slack season for most farmers). Nuts which fall to the ground from cashew trees are collected daily. Thievery can be quite high in densely populated areas. The work is not heavy. Widows in one survey asked for cashew as a crop they could tend with the small amount of family labor that they had available. Another good thing about cashew is that the harvest is right before the rainy season when many poor families need some income for fieldwork.
For those working in areas where cashew is common, one might want to consider helping farmers create marketing associations so that they can sell in quantity and perhaps negotiate better prices with merchants. If superior producing cashew trees are identified, seedlings can be reproduced by cleft grafting of 10cm of new shoot material (from which the leaves have been removed) to young seedlings.