Abstract:

Grapevine cultivation in Zaria, the hub of the production and spread in Nigeria, is threatened by diseases. Diagnostic surveys conducted in June and August, 1998, and January, 1999, to determine the important diseases for studies at the Ahmadu Bello University, Zaria, led to the seasonality studies of the diseases from 1998-2000, which was updated in 2011. The grapevines were found to be plagued by two distinct devastating seasonal health problems with periodicities and associated fungi that were consistent and therefore endemic. Downy mildew, which growers attested they effectively managed prevailed in the dry season; while in the rainy season it was the identified important problem needing research. The disease in the rainy season was associated with many fungi, some of which, were proved pathogenic. For being most preponderant with individual and collective potential to incite the syndrome observed on grapevines in the rainy seasons, Phoma aff. multirostrata, Phomopsis sp., Phoma sorghina and Colletotrichum gloeosporioides were implicated as the main pathogens. The lexicon anthracnose-dieback-blight complex was adopted to reflect the syndrome they incited. In evaluating the interactions among the main pathogens, antagonism was shown only by the influence of Phoma aff. multirostrata which significantly (P < 0.05) reduced the mean severity of the disease incited by the complex of Colletotrichum gloeosporioides and Phoma sp.; while neutrality was common and synergism was most common. In the search for alternative hosts of the main pathogens of the disease complex among horticultural crops, guava and orange isolates of Colletotrichum gloeosporioides significantly (P < 0.05) expanded inoculation spots on grapevines and the berries and incited diseases with mean severity scores that were significantly (P < 0.05) higher than those from the mango, banana and avocado isolates, whose effects were not significantly different from the control. Isolates of Phoma sp. from different parts of the grapevine, mango and sugar apple were not significantly different from one another (P < 0.05) in infectivity and virulence; but were more virulent than all Colletotrichum gloeosporioides isolates. Screening of twenty four grapevine materials for their reaction to field infection of anthracnose-dieback-blight complex, indexed by susceptibility and measured as the mean apparent infection rate, “r”, was zero for ‘Bangalore blue’ and ‘Catawba’. Next were ‘‘Concord’ and ‘Regina’ with “r” values that were not significantly different from one another but were significantly (P < 0.05) lower than the successive ranks. All eight fungicides screened against anthracnose-dieback-blight complex proved efficacious but varied in their efficacy. Metalaxyl + mancozeb (Ridomil MZ 72, Ciba – Geigy) and carbendazim (Bavistin, 50% FL, BASF Aktiengesellschaft) treatments, that were not significantly different from one another, consistently resulted in diseases with the least area under disease progress curves (AUPDC). Bi-weekly spray schedules of metalaxyl + mancozeb (Ridomil M72) and carbendazim (Bavistin) at the manufacturers’ rates with additional spray after each rainy day effectively controlled the disease complex. 12% carbendazim + 63% mancozeb (Camazeb, Jubaili Agrotech Ltd) was as effective as Metalaxyl + mancozeb (Ridomil MZ 72, Ciba – Geigy).

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