Infection and Ultrastructure of Conidia and Pycnidia of Stenocarpella maydis in Maize

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Stenocarpella maydis is the most prevalent ear rot pathogen of maize (Zea mays) in South Africa, the United States, and other countries. Infection and ultrastructure of propagules of S. maydis in maize were observed by light, scanning, and transmission electron microscopy. Two-celled conidia of S. maydis were found in the tissues of husk and kernels. Mycelia colonized inter- and intracellularly in the host tissues. Pycnidia were found abundantly inside the seed tissues of susceptible cultivars; within a single seed, pycnidia propagated preferentially in embryonic tissues. A pycnidium is composed of morphologically different resting spores mingled with some degraded organelles of the host cell. In this study, various enzymatic activities led to cell wall degradation, lacunae in endosperm tissues, and disrupted organelles in susceptible cultivars. In contrast, callose deposition surrounding fungal hyphae was clearly visible in resistant cultivars. Heavy infection was detected by maceration, even though there was no apparent symptom on the seed coat. The saprophytic nature and structurally different forms of propagules could contribute to a long-term survival of this pathogen in the field and during grain storage. Furthermore, S. maydis might pose a threat of diplodiatoxin intoxication to human and domestic animals when infected maize seeds are consumed.