Influence of the Metabolites of three Paecilomyces Species on the Germination and Seedling Development of two Ghanaian Maize Varieties (Abeleehi and Obaatanpa)

Abstract

The mycoflora of two recently-developed maize (Zea mays L) varieties Abeleehi and Obaatanpa have been studied under varying ambient equilibrium relative humidities ERH’s (55, 60, 65, 70, 75, 80, 85, 90 and 95%) representative of the Ghanaian ambient conditions. The potential pathogenicity of selected contaminating fungal species (A. alutaceus, = A. ochraceus, Fusarium, moniliforme, Penicillium digitatum, Peacilomyces carneus, P. puntoni and P. varioti) was also tested under laboratory, field and greenhouse conditions. Finally, the fungal succession or phenology of the species encountered in the rhizosphere and non-rhizosphere soil containing treated maize grains (Abeleehi and Obaatanpa varieties) treated with conidia/mycelium or culture filtrate of the three Paecilomyces species (P. carneus, P. puntoni and P. varioti) was studied. About thirty (30) and twenty-eight (28) species of fungi belonging to the genera Aspergillus, Penicillium, Curvularia, Chaetomium, Cladosporium, Emericella, Eurotium, Fusarium, Paecilomyces, Mucor, Neurospora and Rhizopus were isolated from Abeleehi and Obaatanpa varieties respectively at ERH’s 55-95%. Aspergillus species {Aspergillus candidus, A. effusus, A. fumigatus, A. giganteus, A. niger, A. ochraceus, (= A. alutaceus), A. sulphureus, A. tamarii, A. ustus, A. versicolor, A. wentii and Aspergillus sp) predominated over the others followed by Penicillium (Penicillium brevi-compactum P. critinum, P. verrucosum, P. digitatum, P. expansum, P. funiculosum, P. glabrum and P. nigricans). Fungi belonging to the other genera encountered were Curvularia, Paecilomyces, Chaetomium, Cladosporium, Emericella, Eurotium, Fusarium, Mucor. The species diversity was influenced by grain variety and the ERH at which the grains were stored. Aspergillus flavus was ubiquitous and was encountered in all grains stored at 55-95% ERH. Fusarium monilifonne was isolated from some grains incubated at 65-95% ERH. Xerophilic or xerotolerant fungal species like Aspergillus fumigatus, A. alutaceus ( = A. ochraceus), A. gigunteus, Paecilomyces carneus, P. puntoni and P. varioti were isolated at 55-65% ERH in both grain varieties. The best vegetative growth (radial) of selected species was influenced by both the medium and temperature of incubation. Paecilomyces carneus grew best at 30°C, P. puntoni at 30-35°C and P. varioti at 30°C. All the Paecilomyces species, however, could grow well at 40°C. Aspergillus species tested (A. flavus, A. giganteus, A. alutaceus) grew best at 30°C and remained depressed in growth at 40°C; so did Penicillium digitatum and Fusarium moniliforme. The three Paecilomyces species produced their toxic metabolites in 2 days and their undiluted culture filtrates depressed seed germination of ’Abeleehi’ and ’Obaatanpa’ by 10-75% (depending on fungal species and period of incubation). This inhibitory effect was gradually removed with increasing dilution (up to 1:10v/v). There were varietal differences in the response of the germinating grains to the toxic metabolites of P. carneus, P. puntoni, and P. varioti. Undiluted culture filtrate of the listed three Paecilomyces species also severely depressed length of the emerging radicles of ‘Abeleehi’ and ‘Obaatanpa’ by 45-90% but this inhibition was gradually removed by increasing dilution of the culture filtrates (up to 1:10V/V dilution). The inhibition of seed germination and radicle development by culture filtrate of the three Paecilomyces species was not confined to maize only as their adverse effect on seed germination and radicle development was reproduced in vitro using tomato (Lycopersicon esculentum Mill var. Owusu-Bio and Wosowoso) and pepper (Capsicum annuum L). In this instance, the inhibitory principle was still potent even at 1:10V/V dilution level. Culture filtrate of Aspergillus alutaceus ( = A. ochraceus) at the highest concentration depressed seed germination of Abeleehi and Obaatanpa varieties by 50-70% and reduced radicle length by 60-90%. The inhibitory effect was gradually removed by increasing dilution (up to l:10v/v). Similarly, F. moniliforme and P. digitatum had the same deleterious effect on germination and radicle development of ’Abeleehi’ and ’Obaatanpa’ maize varieties. Seed germination was depressed 50-70% and radicle length by 40-90% when the undiluted culture filtrate of F. moniliforme and P. digitatum were applied to the grains. In all instances, the inhibitory effect was gradually removed by increasing dilution o f the culture filtrates. There were varietal differences between the three Paecilomyces species in their effect on vegetative growth and dry matter accumulation of Abeleehi and Obaatanpa maize varieties. Metabolites of P. carneus, P. puntoni and P. varioti variably depressed plant height, leaf width, leaf length, dry matter accumulation, (dry weight of root and shoot systems) as well as chlorophyll a and b contents of Abeleehi and Obaatanpa varieties cultivated in the field and under green house conditions. The maize cobs obtained from the field plants infected with Paecilomyces species were diminutive with fewer and smaller grains in the cob as compared to the control. Culture metabolites of P. carneus, P. puntoni and P. varioti reduced by 2-3 times diameter of roots of the seedlings of Abeleehi and Obaatanpa although the endodermis and pericycle were clearly formed and demarcated in both the control and treated seedlings. The pith parenchyma was thinly lignified and 2-3 times narrower in diameter in the treated plants exposed to the three Paecilomyces species; pro - and metaxylem vessels were about 2 times wider in the control seedlings and the phloem and xylem regions o f the roots o f the treated plants were reduced in number and size. Maize grains (Abeleehi and Obaatanpa varieties) inoculated with three Paecilomyces species influenced the rhizosphere mycoflora and their succession profile. Generally, the species of fungi that were stimulated, depressed or eliminated varied from one grain variety to another growing either in the field or under greenhouse conditions. Aspergillus flavus, A. niger, A. alutaceus, ( = A. ochraceus) and A. versicolor remained viable in the rhizosphere soil inspite of the presence of the inhibitory principles exuding from the Paecilomyces species; Penicillium citrinum could tolerate the same metabolites while P. digitatum, P. brevi - compactum did not grow very well in competition with the three Paecilomijces species. Population o f other fungi encountered belonging to the genera Cladosporium, Fusarium, Mucor, Scopulariopsis, Trichoderma and Yeast declined with time. Cladosporium herbarum, F. moniliforme, Mucor sp, Rhizopus otyzae and T. viride survived in the treated soil in competition with the metabolites of the Paecilomyces species. The practical implication of these findings are discussed and future studies suggested.