Seed-Borne Fusarium sp.Causing Damping-off of Pinus tecunumanii Seedlings in Tanzania

Abstract

Seed-borne fungi are among the most serious groups of pathogens, causing significant losses in both quantity and quality of Pinus seeds. Conifer nurseries serve as reservoirs for a diverse suite of native and exotic pathogens, e.g., Fusarium and Phytophthora spp. These pathogens can move undetected through asymptomatic seedlings and infected nursery stock which can cause mortality in field-planted seedlings and risk of pathogen introductions into plantations. Occurrence of Fusarium spp. is one of the problems, most limiting to growth of seedlings in nurseries. The pathogen can be transmitted via seeds and causes damages to the seedlings during pre- and post-emergence stages. The present study aimed to perform seed health testing of P. tecunumanii through blotter test method and to identify seed-borne pathogens which affect the species and cause damping-off disease. Seed and symptomatic seedlings samples were collected from four different seed lots namely; Private Forestry Programme (PFP), Makungu, Mafinga, and Kilolo. The results came out with four major fungi species which are; Aspergillus niger, Penicillium sp, Rhizopus sp and Fusarium sp. Fusarium sp was suspected to be the major seed-borne fungi which causes damping-off disease in young seedlings. Identification of Fusarium started at the species level based on morphological and molecular characteristics. Morphological characterization was performed based on a key specific to Fusarium spp. Whereas, molecular identification was carried out by amplification and sequencing of the regions from internal transcribed spacer (ITS) and the elongation factor using specific primers for F. circinatum. The results tasted negative implying that the suspected Fusarium is not circinatum species but, based on the results of molecular identification using universal primer pairs ITS1 (forward) and ITS4 (reverse), isolate (TZA01_DM) is identical to the fungal pathogen Fusarium sp. Sequence alignment fungal nucleotides amplified with the database at Genebank showed that, pathogenic isolate (TZA01_DM) had the closest homology of 100% with Fusarium oxysporum from France, Iran, India, Ghana and Switzerland, also has the 100% similarity index with Fusarium mundagurra (China), Fusarium concentricum (China), and Fusarium annulatum (India and Spain). The pathogenicity test was conducted through the contact of the seeds with fungal culture for 12 hrs., followed both the isolates were pathogenic to the seeds of Pinus spp., causing a reduction in the percentage of emergence and seedling damping-off. Different three fungicides were used namely Multipower Plus (78% WP), Ridomil Gold (68 WG), and Farmerzeb (80 WP), to control the mycelial growth rate of Fusarium sp in vitro and in vivo and significant results were obtained through the analysis of data using one-way ANOVA (analysis of variance) and Statistical Package for the Social Sciences (SPSS). In vitro results indicated Farmerzeb to be the best compare to the rest of the fungicides. Under in vivo germination percentage for treated and untreated controls had significant difference. Results also showed no phytotoxicity observed throughout the experiment. This is a first report in Tanzania to identify different fungi species that affect forest seeds and went further to control Fusarium obtained from such seeds through available fungicides. Moreover, the study laid the foundation for some more research to be conducted on forest seeds. The significance of the study will be important in developing knowledge on the proper control of Fusarium sp. in order to minimize the infestation rate and achieve sustainability of forest plantations for future use.