Surveys were carr~ed out to determine the prevalence of plgeonpea d~seases In the major plgeonpea growlng areas of Asla Afrlca and the Amerrcas between 1975 and In lnd~a suwcys In eleven states revealed that wllt storll~ty mosalc Phytophthora bl~ght Macrophomlna stem canker and yellow m0sa.c were econorn~cally lmportant dtseases Other dlseases were of rnlnor Importance Disease problems In Bangladesh Malays~a and Nepal were of less Importance In Afrlca wllt was a serlous d~sease tn Maiaw~ 136 3%). Tanzanla (20 4%1 and Kenya (15 9%) Leaf spot In Kenya and Malaw1 dnd powdery m~ldew In Kenya, Tanzan~a and Zamb~a were Important Other dlseases were not economically Important In the Amerlcas wltches broom Phoma stem canker and rust were the Important drseases Annual crop losses due lo the comblned effecl of w~lt and stertllty rnosalc d~seases In lnd~a were estlrnated to be worth dbout US$ 113 mlll~ons In Afrlca the estimated losses from wtlt disease alone were over US$ 5 rnlll~ons annually

This paper tries to illustrate how beneficial interactions between crops can be exploited to increase the overall output of a cropping system. Growth studies with sorghum/pigeonpea and millet/groundnut are described to show how intercropping systems can achieve much larger yields than sole crops by using environmental resources more fully over time or more efficiently in space. Data from moisture stress studies are presented to illustrate that these advantages of intercropping can be even greater under stress conditions.

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