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Relation of light and temperature to the sporulation of Alternaria tomato (Cke.) Weber

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Title:Relation of light and temperature to the sporulation of Alternaria tomato (Cke.) Weber
Authors:Aragaki, M. (Minoru)
Keywords:Fungi -- Hawaii
Alternaria tomato
Plant spores
Date Issued:1963
Abstract:Reproduction is one of the physiological processes of fungi which is of great interest to mycologists, plant pathologists, and other biologists. With fungi, as with other plants, the development and the morphology of sexual reproductive structures are of great importance in identification and classification. In general, fungal reproduction is divided into two parts - sexual and asexual. Spores from the sexual and asexual reproductive bodies function to disperse the organism to new areas or substrates and may also allow the fungi to survive unfavorable conditions. Many fungi are either unable to produce or are not known to produce sexual reproductive bodies. These fungi are placed in the class Fungi Imperfecti and are classified according to their asexual reproductive structures. Although spores from these asexual reproductive structures are not heavily relied upon for classificatory purposes, they are important biologically in that they allow for the dispersal of the species and for survival during unfavorable conditions. Many of the fungi that are commonly encountered, e.g., Aspergillus spp., Penicillium spp., Cladosporium spp., Rhizopus spp., Fusarium spp., etc., do not develop sexual reproductive bodies under normal conditions and thus are difficult to identify and classify. These fungi, however, produce asexual fruiting structures which can be used in their identification and classification under the admittedly artificial but useful Saccardoan system. Asexual reproduction is frequently many times more prolific than sexual reproduction. with the aid of slight genetic changes and selection pressures, the species is able to survive adverse conditions. The very fact that these are common fungi attests to their reproductive capabilities. Under environmental conditions favorable for growth, many fungi of the Ascomycetae and Phycamycetae reproduce asexually, whereas under less favorable conditions for vegetative growth there is a greater tendency for sexual reproduction to take place. With many phytopathogenic fungi, asexual sporulation takes place continuously or in successive waves under favorable conditions. Host of them are known primarily by their asexual stages, and in tropical or subtropical areas with year-round favorable growing conditions, perfect stages of many Ascomycetae and Phycomycetae are not common (55). Asexual sporulation of most fungi occurs readily enough under natural conditions but in vitro sporulation can be very poor. In addition some fungi lose the ability to sporulate in culture, more or less rapidly, with repeated transfer. Much work has been done in the field of fungal sporulation, but because of the heterogeneity of the test organisms and the inherent complexities of the reproductive processes very few generalizations can be made. Among the factors which have been studied and reviewed by Cochrane (SO), Hawker (52), and Lilly and Barnett (53) are light (radiation), temperature, water relations, oxygen-carbon dioxide relations, pH, carbon nutrition, nitrogen nutrition, vitamins, and specific reproductive hormones. Much of the effort in the study of sporulation was made to obtain spores in quantity. Large numbers were required for spore germination processes, various pathological processes, epidemiology, genetics of fungi, or for making cytological studies of the spores themselves. Such studies have resulted in numerous methods of inducing fungal sporulation. Most of these were empirical in nature and as soon as a certain technique proved satisfactory for sporulation very little further effort was made to study the fundamental factors involved. The present study was also started in this vein, i.e., spores of Alternaria passiflorae Simmonds were needed for epidemiological studies of passion fruit brown spot. However, this fungus, like many large-spored members of the genus (10, 34, 37, 44), sporulated very sparsely under laboratory diurnal light and temperature fluctuations and did not sporulate in incubators at temperatures ranging between 15° and 35°C. During the summer of 1960, a collection of passion fruit brown spot was made and isolations revealed the causal organism to be an Alternaria. sp. distinct from A. passiflorae. This species was subsequently identified to be Alternaria tomato (Cke.) Weber. A. tomato is further distinguished by its capacity to form spores in abundance in distinct sporulating zones (Fig. 1). These zones are dark in color, primarily consisting of conidiophores and conidia with sparse or no aerial vegetative mycelia. Light-colored zones of abundant aerial mycelia, bearing moderate numbers of conidiophores and conidia are also formed. These zones correspond to diurnal changes in the laboratory. Under continuous illumination it was found that, although dark conidiophores were formed almost to the exclusion of hyaline vegetative mycelia, actual sporulation did not take place (Fig. 2). Under continuous darkness, growth consisted of aerial mycelium bearing conidiophores and conidia. When cultures, which had been growing under continuous illumination, were transferred to darkened incubators, spores formed on virtually all of the conidiophores (Fig. 3). A study of cultures of A. tomato grown under continuous illumination at several constant temperatures revealed that the fungus sporulated readily at 20 C under continuous illumination but produced conidiophores without spores at 25° or 30°C. This phenomenon of high-temperature X light inhibition of sporulation has not been studied. The present study was thus undertaken to look into the factors influencing sporulation of A. tomato.
Thesis (Ph. D.)--University of Hawaii, 1963.
Bibliography: leaves 44-48.
iv, 48 leaves mount. ill., mount. diagrs., tables
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Appears in Collections: Ph.D. - Botany

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