Seed-borne fungi of herbs cultivated in South Africa and evaluation of non-chemical seed treatments to control Alternaria sp. on coriander

Abstract

Seed-borne mycoflora associated with eight herb seed species, viz. basil (Ocimum basilicum L.), chives (Allium schoenoprasum L.), coriander (Coriandrum sativum L.), dill (Anethum graveolens L.), parsley [Petroselium crispum (Mill.) Fuss], sage (Salvia officinalis L.), thyme (Thymus vulgaris L.) and wild rocket [Diplotaxis tenuifolia (L.) DC.], and their effects on seed germination were studied. Studies on the pathogenicity of isolated seed-borne Alternaria tenuissima (Kunze) Wiltshire were also performed; whereof evaluations of non-chemical methods were conducted to control the aforementioned pathogen. Seed health tests detected ten genera of fungi associated with herb seed lots, which included Alternaria, Aspergillus, Cladosporium, Curvularia, Epicoccum, Fusarium, Penicillium, Rhizoctonia, Rhizopus and Trichoderma. It was observed that Alternaria, Aspergillus, Fusarium and Penicillium were the predominant fungi. This study represents the first record of seed-borne fungi associated with herbs of the Apiaceae and Lamiaceae plant families in South Africa. Findings of seed germination tests showed that all herb seed lots were above their minimum acceptable levels, except for seed lots of dill and wild rocket. In addition, correlation analysis showed that incidence of seed-borne fungi was positively correlated with the number of diseased seedlings raised from the aforementioned seed lots (r= 0.239, p<0.01). Pathogenicity tests showed that the seed-borne fungus, A. tenuissima, was both seed-transmitted and pathogenic on coriander. In -vitro screening tests to investigate the antifungal effects of plant extracts of Allium sativum L, Carica papaya L, Datura stramonium L, Lantana camara, Tagetes minuta and Zingiber officinale Roscoe were conducted against pathogenic A. tenuissima. Based on the agar infusion method, it was observed that most acetone and ethyl acetate extracts effectively inhibited growth of A. tenuissima when applied at low concentrations, which had minimum inhibitory concentration (MIC) values of ≤5 mg/ml. However, most water extracts demonstrated poor antifungal activities; thus, recorded high MIC values (>10 mg/ml). From the antifungal tests using the disc diffusion method, the ethyl acetate extract of Allium, acetone extracts of Datura and Zingiber, and the water extract of Lantana were selected for further evaluations in the greenhouse as they demonstrated good antifungal activities against A. tenuissima. Furthermore, a preliminary in vitro study was conducted to examine the optimum hot water treatment temperature-time combination that effectively controls A. tenuissima associated with coriander seeds. Findings of this study showed that seeds soaked in hot water at 54ºC for 15 mins resulted in a considerable reduction of the incidence of A. tenuissima with minimal effect on seed germination. However, soaking coriander seeds at temperatures above 54ºC significantly lowered seed germination. Naturally infected coriander (cultivar American long) seeds treated with plant extracts of Allium, Datura, Lantana and Zingiber; hot water at 54ºC for 15 mins and biological control agents of Bacillus subtilis (Ehrenberg) Cohn and Trichoderma harzianum Rifai were evaluated for their effects on seed germination, seedling emergence and incidence of Alternaria leaf spot disease incited by A. tenuissima. This study showed that all seed treatments effectively lowered the incidence of A. tenuissima on coriander, which resulted in improvement of percentage seed germination. However, Datura extracts negatively affected seed germination as it resulted in higher numbers of abnormal seedlings. Greenhouse experiments showed that sowing treated seeds significantly increased seedling emergence and seedling growth. Thus, seedlings raised from treated seeds were significantly longer and had broader leaf surface area, which contributed to higher seedling fresh and dry mass compared to untreated seeds. The incidence and severity of Alternaria leaf spot disease was less pronounced on coriander seedlings raised from coriander seeds treated with extracts of Allium, Zingiber and the biological control agent, Bacillus sp.