Aetiology and Control Citrus Gummosis Disease the University Agriculture Station, at of Ghana Research Station, Kade

Abstract

The citrus gummosis disease is a major problem facing the Agriculture Research Station of the University of Ghana at Kade. The disease is the cause of death of citrus plants on the station, hence this work initiated to investigate the cause(s), possible contributing factors and its control on the station. A field survey conducted from January to June 1996 gave 36.2 -79.9% incidence (Avg. 62.6%) on six orchards of the station and an average severity index of 1.2 on a 0-5 scale. A correlation analysis proved that both disease incidence and severity increased with age (r = 0.705 and 0.876 respectively). Plants with ages above twenty years, were more severely affected by the gummosis disease. Plants, established with rough lemon rootstock were more susceptible and severely affected by the gummosis disease than plants established with Cleopatra mandarin or rangpur lime rootstocks. Infection was more freguently located on the scion (73-85%) than on the rootstock (15-27%) materials. Termites were observed to carry soil particles which may possibly be infected with the infective units of the pathogen up the plants. Their activities probably result in wounding the surfaces of the bark by abrasion, for the pathogen in the soil to infect the plants. Phytophthora parasitica, the hitherto known pathogen of the disease in Ghana could not be isolated, after several attempts including the use of selective media. Rather, Diplodia natalensis Pole Evans, one of four fungi isolates obtained from the isolation processes, induced gummosis disease symptoms in artificially inoculated rough lemon seedlings. Koch's postulates were satisfied. Fusarium solani and two other unidentified fungi isolates were unable to cause the disease sysmptoms in the pathogenicity test. In a rootstock resistance screening work, with D. natalensis as the test fungus, Volkameriana (C. volkameriana Pasg.) seedlings oozed the least amount of gum and had lesions which were smaller in size than those on the standard rough lemon seedlings. Seedlings of Swingle citrumelo (C. paradisi Macf.X C. trifoliata) and Obuasi {C. sinensis (L) Osbeck}, a local cultivar oozed gum profusely and Swingle in addition, developed the largest lesion as compared to rough lemon. Ridomil (metalaxyl) at 40g and 60g ai/L, aliette (phosetyl aluminum) at 200g and 400g ai /L and phosphorous acid (Foli-r-fos) also at 150g and 300g ai/L were effective in controlling the disease on the field. Ridomil however, was more effective than the other two fungicides. Two applications of these fungicides within a year was also more effective than single application. Bordeaux mixture (1:4) and bavistin (50% carbendazim) at lg and 2g ai /L were ineffective against the disease.