Use of Canavalia ensiformis as Green Manure for Managing the Root Rot and Vine Decline Complex in Melon

Root rot and vine decline (RRVD) is one of the main root diseases of melon, causing significant production losses due to the isolated or combined action of different phytopathogenic fungi. The absence of registered chemical control methods in Brazil reinforces the need for alternative integrated management strategies. This study aims to evaluate the enzymatic responses of Jack Beans plants inoculated with M. pseudophaseolina (Experiment I) and the effectiveness of crop rotation cycles between melon (ME) and Jack Bean (JB) on RRVD and soil-borne pathogens (Experiment II). In Experiment I, the activity of chitinase (CT), β-1,3-glucanase (BG), and phenylalanine ammonia-lyase (PAL) were assessed at five evaluation periods. In Experiment II, nine rotation cycles involving JB and ME, grown in naturally infested soil, were evaluated for disease incidence, severity, and fungal isolation frequency. In Experiment I, inoculated plants showed increased CT, BG, and PAL expression compared to non-inoculated plants. In Experiment II, the control treatment showed no disease incidence or severity, whereas all other rotation cycles exhibited 100% incidence and severity ranging from 4.5 to 4.9. The incorporation of JB demonstrated a suppressive effect on important pathogens associated with branch decline, reducing the frequency of Macrophomina spp. isolation (0–4% compared to 16% in the treatment without JB) and delaying the root penetration of Monosporascus spp. after two consecutive cycles. However, this management also favored the multiplication of Fusarium spp. in all treatments with JB incorporation.