Severity of Curvularia Stem Blight Disease of Cassava in West Africa.

In diagnostic surveys, Curvularia stem blight affected 9, 13, and 38% of cassava fields, respectively, in Benin, Ghana, and Nigeria. Disease incidence (number of plants with visible symptoms per total sampled) ranged between 0 and 80%, and severity (number of lesions) between 2 and 25 lesions per stem. In greenhouse studies, the fungus inhibited shoot growth depending on the degree of bud colonization, such that when buds were completely colonized, they failed to sprout. Partially colonized buds sprouted, but depending on genotype, overall growth was reduced 20 to 50% compared with healthy stems. Shoot growth for all artificially inoculated cultivars was consistently lower than for the respective noninoculated plants, and they suffered up to 50% leaf abscission. In two field localities, shoot sprouting for cultivars TMS 30572 and Odongbo was reduced 4 to 18% and 26 to 58% compared with noninoculated stems.

Prevalence and Severity of Nattrassia mangiferae Root and Stem Rot Pathogen of Cassava in Bénin.

Root rot pathogens were found through diagnostic surveys in all departments (regions) of Bénin, West Africa, to affect 86 to 100% and 96 to 100% of cassava fields during the dry and rainy seasons, respectively. Disease incidence in individual fields ranged between 0 and 53%, and averaged 16 to 27% per department. Nattrassia mangiferae was consistently the most frequently isolated root rot pathogen (56% in the dry season and 22 to 52% in the rainy season). Pathogenicity of N. mangiferae was confirmed on four cultivars of cassava using stem cuttings and storage roots. For all four cultivars, N. mangiferae significantly reduced the number of roots. Lesions (3 to 15 cm long) formed on the lower stem portion of all inoculated plants, whereas control plants remained symptom free. On storage roots, the disease profile was similar to that formed on stem cuttings. Other root rot pathogens detected during the dry season were Macrophomina phaseolina (14.2%), Fusarium spp. (11.8%), Botryodiplodia theobromae (7.7%), and Pythium spp. (2.9%). During the rainy season, Fusarium spp. were the second most commonly isolated root rot pathogens in three departments (Atlantique, Borgou, and Mono). In Oueme and Zou, B. theobromae was the second most isolated root rot pathogen (ranging between 24 and 28%) during the rainy season. During the same season, Pythium spp. were pronounced in Borgou (18%), followed by Mono (11%), Atlantique (9%), Atacora (8%), Oueme (5%), and Zou (6%). Results of the study are discussed with a view to creating awareness of the destructive power of N. mangiferae, a hitherto poorly recognized root rot pathogen of cassava in Bénin and West Africa in general.

First Report of Macrophomina phaseolina Causing Pre-harvest Cassava Root Rot in Benin and Nigeria.

In diagnostic surveys conducted in parts of Benin and Nigeria to determine the incidence of pre-harvest cassava root and stem rot during the dry season, Macrophomina phaseolina (Tassi) Goidanich constituted 14.2 and 18.7% of the total fungi (n = 201) associated with cassava root and stem rot from Benin and Nigeria (1). Pathogenicity of M. phaseolina on cassava was tested with cv. Agric. Inocula for pathogenicity tests were prepared by incubating 5-mm-diameter mycelial plugs for each of five isolates (Mp 1 to Mp 5, all collected from Benin) with 500 ml of autoclaved, sterilized, dehusked rice seed for 14 days at 30°C. Five 30-cm-long stem portions per isolate were cut from healthy cassava, surface disinfested in hot water (52°C, 5 min), and planted into 1-liter pots containing autoclaved, sterilized sand mixed with 10 ml of air-dried inoculum. Five plants per isolate similarly treated but not inoculated served as controls. Plants were watered once a week, and maintained in a greenhouse under natural light at 28 to 30°C. Lower leaves of inoculated plants gradually wilted, usually preceded by chlorosis, and brown to black lesions formed on the lower stem portions of some roots. Control plants remained asymptomatic. Plant height and percentage of leaf wilt (determined by counting the number of leaves wilted per plant and dividing by the total number of leaves per plant) were measured on a weekly basis for 8 weeks for each of the control and inoculated plants. At the end of 8 weeks, lesion length on the lower stem was measured. There were significant differences (P < 0.05) in length of the lesions and percentage of leaf wilt induced by the different isolates of M. phaseolina. Isolate Mp 1 induced the longest lesion (7.2 cm), followed by Mp 4 (4.1 cm), Mp 3 and Mp 5 (3.8 cm each), and Mp 2 (1.2 cm). Mp 4 induced the highest percentage of wilted leaves (53%), followed by Mp 1, Mp 3, and Mp 5 (30%), and Mp 2 (10%). All five M. phaseolina isolates (except Mp 3) reduced plant height, compared with control treatments. M. phaseolina was isolated from all infected plants, and the identification was independently confirmed by the International Mycological Institute, Surrey, UK. This is the first report of M. phaseolina causing pre-harvest cassava root rot in Benin and Nigeria. Reference: (1) W. Msikita et. al. Plant Dis. 81:1332, 1997.

First Report of Nattrassia mangiferae Root and Stem Rot of Cassava in West Africa.

During part of the dry season in 1996 (November to December), surveys were made for incidence of root and stem rot in 99 fields of cassava (Manihot esculenta Crantz) randomly selected between latitudes 6°36'N and 7°49'N in Benin (79 fields) and Nigeria (20 fields). Root rot was observed in 65 fields in Benin and 15 fields in Nigeria. Disease incidence ranged from 0 to 54%. A total of 201 samples of wilted and/or dead plants were collected for laboratory analysis. Infected root and stem portions (0.5 to 1 cm) were cut out, surface disinfested (10 min) in 10% bleach (0.6% sodium hypochlorite), rinsed in sterilized distilled water, and cultured on potato dextrose agar acidified to pH 4.5 with 0.4% (vol/vol) lactic acid. Cultures were incubated at 25°C, under 12-h day length provided by cool-white fluorescent lamps. After 1 week, mycelia, conidiophores, and conidia were observed at ×30 to ×40 magnification under a compound microscope. Out of the 169 symptomatic samples collected from Benin, nine fungal genera were isolated: Aspergillus spp. (1% of fungi observed), Botryodiplodia theobromae Pat (7.7%), Fusarium spp. (11.8%), Macrophomina phaseolina (Tassi) Goidanich (14.2%), Nattrassia mangiferae (Syd. & P. Syd.) B. Sutton & Dyko (56.2%), Penicillium spp. (0.6%), Pythium spp. (2.9%), Rhizopus spp. (1.7%), and Trichoderma spp. (2.4%). One percent of the fungi isolated did not sporulate in culture and were not identified. Out of the 32 samples collected from Nigeria, four fungal genera were identified: N. mangiferae (40.6%), B. theobromae (28.1%), M. phaseolina (18.7%), and Fusarium spp. (12.5%). Since N. mangiferae was isolated with the highest frequency, its pathogenicity was tested on cassava (cultivars Agric, Ben 86052, Dessa 88, Tchukunochi, and TMS 30572). Two weeks prior to the experiment, inocula for pathogenicity tests were prepared by incubating 5-mm-diameter mycelial plugs of N. mangiferae with 500 ml of autoclaved rice seed for 10 days at 25°C, followed by air drying in a laminar flow hood for 2 days. Five 30-cm-long stem portions were cut from healthy plants of each cassava cultivar, surface disinfested in hot water (52°C, 5 min), and transplanted into sterilized (autoclaved, 1 h) sand in 1-liter pots to which 10 ml of the N. mangiferae-colonized rice inoculum had been added. There were five control stems for each cultivar, similarly treated, but not inoculated. Plants were maintained in a greenhouse under natural light at 28 to 30°C. Thirty days after planting, plant height, lesion length, and number of shoots and roots were recorded. For all five cultivars, N. mangiferae significantly (P < 0.05) reduced plant height and number of shoots and roots, compared with control plants. Lesions (3 to 15 cm long) formed on the lower stem portions of all inoculated plants, resulting in variable degrees of wilting of the infected plants. Two of the cultivars (Agric and Ben 86052) died 3 weeks after planting. Control plants remained asymptomatic. N. mangiferae was consistently reisolated from infected plants, and the identification was independently confirmed by the International Mycological Institute, Surrey, UK. Scytalidium sp., a synamorphic state of N. mangiferae (2), was reported to cause up to 85% cassava root yield loss in South America (1). This is the first report of N. mangiferae causing cassava root and stem rot in West Africa. References: (1) Anonymous. Annu. Rep. Cassava Prog., CIAT Working Doc. No. 116:97, 1992. (2) B. C. Sutton and B. J. Dyko. Mycol. Res. 93:466, 1989.

First Report of Curvularia lunata Associated with Stem Disease of Cassava.

During surveys covering 60 cassava (Manihot esculenta Crantz) fields, randomly selected (between latitude 4°55'N and 8°16'N) in south Ghana, and 27 fields in southeast (between 4°50'N and 7°56'N) Nigeria, 8-month-old or older stems of some cassava genotypes were found to be covered by grayish brown lesions, predominantly on lignified portions of stems. Field disease incidence ranged from 0 to 80%, and severity from no disease to highly affected (>15 lesions per stem). To identify the pathogen, infected stem portions were cut out, surface disinfected, and cultured on potato dextrose agar (PDA) acidified with 0.4% (vol/vol) lactic acid. After 1 week, mycelia, conidiophores, and conidia were observed under a microscope, and the pathogen was identified as Curvularia lunata (Wakk.) Boedijn (confirmed also by the International My-cological Institute, Surrey, U.K.). To complete Koch's postulates, stem pieces of four cassava cultivars (Agric, Tchukunochi, TMS 30572, and Ben 86052), were disinfected in hot water (52°C for 5 min), transplanted in sterilized sand, and maintained in a greenhouse under natural light at 28 to 30°C. Before planting, five stems were wound inoculated (sliced with an epidermal scalpel) just above nodes, and a 5-mm-diameter PDA mycelial plug of C. lunata was applied to each wound or directly to unwounded nodes. Stems were then kept in a plastic bag for 24 h before planting. For each cultivar, five control stem pieces were similarly wounded but not treated with PDA plugs. All plants were maintained under >90% relative humidity. Control plants remained symptom-free whereas lesions and bud necrosis similar to field symptoms were observed on all inoculated plants within 1 month. Symptom development was quicker (2 to 3 weeks) on wound-inoculated than on nonwounded stems. Mortality of artificially wound-inoculated buds ranged between 30 and 100%, depending on genotype and manner of inoculation. Artificially infected stem and bud portions plated on PDA consistently yielded C. lunata. Bud sprouting of naturally infected cuttings was monitored over a period of 4 weeks after stem planting. When buds were completely colonized, sprouting was completely inhibited. However, partially colonized buds sprouted, but growth was reduced by 20 to 50% (depending on genotype), compared with healthy stems. This is the first report of C. lunata pathogenic on cassava.