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Seed-borne Infection of Colletotricum Truncatum (Schw.) Andrus and Moore in Soybean Seeds and its Control
Dissertation Abstract:
Colletotrichum truncatum is a recognized soybean seed pathogen, causing pre- and post-emergence damping-off of seedlings and seedling hlight. Detection and isolation of this pathogen were carried out to understand its nature. In the examination of seed-borne microorganisms of soybean (Glycine max (L.)) Merill cultivar Palmetto, SJ4, and SJ5 collected from Malaysia and Thailand. nine genera composed of 13 fungi species were associated with the seeds. Among these fungi. C. truncatum incidence was recorded in the range of 2.5-6.5 percent. The blotter method proved more suitable for detecting C. truncatum than agar plate method.
It was confirmed that C. truncatum infection reduced seed quality and seedling vigor. Infestation of soybean seeds with conidial suspension or culture filtrate· and infestation of soil with culture suspension significantly reduced percentage of seed germination, seedling emergence, and seed ling height in in vitro and glasshouse studies. The fungus caused seed rot, pre- and post-emergence damping-off and lesions on the hypocotyls and cotyledons of young seedlings. Seeds infected by C. truncatum contained more so luble protein but less soluble carbohydrate than the healthy seeds. AIthough the electrophoretic pattems were qualitatively identical between infected and healthy seeds, the relative amount still varied. This findin g supported the observation that C. truncatum resulted in seed deterioration and reduced genninability. In addition, infected seeds had lower activity of soluble isozymes (esterase. peroxidase, acid phosphatase, and alkaline phosphatase) than healthy seeds.
Soybean plants were found to be more susceptible to C. truncatum at maturity. Inoculation at different stages (R1, R3, RS, and R7) of plant growth showed that the recovery of seed-borne C. truncatum was highest at R7 (17.56%), followed by RS (10.38%), R3 (9.19%), Rl (7.69%), and the uninoculated control (3.44%). Component plating of individual seed parts confirmed that the fungus was internally-borne and well-established within the seed coat. Therefore, this study positively showed that C. truncatum can be transmitted from mother plant to developing seeds. The pathogen was also transmitted from infected seeds to seedlings. C. truncatum produced reddish brown to light brown lesions on infected cotyledonary leaves, hypocotyl, and epicotyl regions of germinating seeds. Lesions became dark brown to black as the disease progressed. producing numerous black acervuli in the center. Severe infection resulted in pre- and post-emergence mortality and stunting of young seedlings. Using the seed ling symptom tests, seed transmission was assessed to be 83 percent and 59 percent on soil and sand, respectively.
Light and scanning electron microscopy confirmed that the fungus was internally-borne within an infected soybean seed. Abundant inter- and intra-cellular hyphae were observed in all the three layers of the seed coat (palisade cell, hourglass cell, and parenchyma cell layers) and in the hilar tracheids of the seed. Acervuli primordia were produced in the hourglass cell layer and endodermis cell. Fruiting structures were later produced on the seed surface. Parenchyma cells appeared collapsed and macerated.
Six of the 12 isolates of fungi and bacteria that were detected and isolated from soybean seeds were found prom ising as biocontrol agents against C. truncatum in dual culture. These were Chaetomium globosum, Curvularia lunata, Fusarium moniliforme, F. oxysporum, F. semitectum, and Bacillus sp.
Chaetomium glohosum, F. oxysporum, and Bacillus sp. were selected for further study as seed treatment in comparison with six other commonly used fungicides. All seed treatments significantly reduced the incidence of C. truncatum in the infected soybean seeds in in vitro test. However, in glasshouse studies, results showed that only the fungicidal seed treatments (Delsene, Benlate, Captan, and Thiram) improved the rate of seed germination and seedling emergence.