Seasonal changes of secondary compounds in the phloem sap of cassava in relation to fertilisation and to infestation by the cassava Mealybug

Abstract

The aim of the present study was to determine, by a field experiment, whether the seasonal variations in the levels of cyanide and rutin in the phloem, secondary compounds potentially implicated in cassava resistance to the cassava mealybug (Phenaccocus manihoti), could be influenced by different kinds of soil fertilisation (manure, NPK, KCl, lime, mulch). It was found that mean cyanide and rutin contents varied significantly with season, soil treatments and infestation. For each soil treatment, the highest contents of these compounds on uninfested plants was registered in the main dry season (Jul–Aug) for cyanide and at the end of the dry season (Jul–Aug) and beginning of the rainy season (Oct–Nov) for rutin. The infestation factor induced an increase in cyanide and rutin contents for 43 combinations of period and soil treatment among 48 in total. For cyanide content (phagostimulant function in mealybug), this increase appeared significant at the end of the rainy season (Apr–May) and in the main dry season (Jul–Aug) for control and KCl fertilisation. For rutin content (an unfavourable substance for the insect) significant increase was induced by infestation in each period for manure and mulch and especially in the main dry season (Jul–Aug) and at the beginning of the rainy season (Oct–Nov) for other soil treatments. Concerning the seasonal variations in the level of rutin, these results showed that mulch and manure are the best fertilisers to increase cassava resistance against mealybugs.