EFFECT OF CASHEW CLONE USED AS ROOTSTOCKS AND SCIONS ON GUMMOSIS INCIDENCE IN CASHEW PLANTS

The expansion of early dwarf cashew (EDC) crop in Brazil has promoted a reduced genetic diversity of cashew genotypes used to both as scion and root-stock. This may contribute to enhance crop vulnerability to some biological harm such as the infection by the gummosis fungus agent, Lasiodiplodia theobromae, the most important disease of this plant in semi-arid north-eastern Brazil. It has been estimated that half a million graft plants of EDC are yearly produced in Brazil, most of them obtained by grafting over seedling from CCP 06 clone as root-stock. The only means of managing gummosis is by the use of genetically resistant clones. However, the lack of studies on the scion and stock genotypic interactions has contributed to outbreak successive gummosis epidemics. This study was developed in order to evaluate the effect of different combinations of EDC clones used as scion and stock on gummosis incidence. Root-stocks, prepared from seeds of six open pollinated plants, were grafted with two commercial clones: CCP 75 (susceptible) and BRS 226 (resistant). The field trial was arranged under condition of high disease pressure, in a 2 by 6 factorial scheme in a completely randomized block design with four replications and nine plants per plot. Disease incidence (I) and severity (S) (scale 0 to 4) was monitored for three years. On the third year, gummosis incidence and severity were higher in CCP 76 (I = 97% and I = 2.10) than in BRS 226 (I =33% and I = 0.36), independently of root-stock. CCP 06 clone was the only one root-stock which reduced both incidence and severity of gummosis independently of the scion. This clone was able to reduce susceptibility of CCP 76 as canopy.