Studies on the Parasitism of the Rust of Acacia Confusa MERRILL, Maravalia hyalospora (SAW.) DIET

Abstract

The present investigation was performed cytologically in order to find any difference in the reaction of the rust of Acacia confusa to host cells at different parts of the same phyllode as well as to those of different phyllodes with different maturity.(1) Susceptible region, viz., young, growing portion of the phyllode.(a) The entering hyphae produced from appressoria usually invade through stomatal apertures, less frequently piercing cuticle directly (fig. 3), and form a substomatal vesicle (fig. 1, 2), from which haustoria are produced in the adjacent cells. After a while a few primary hyphae are also formed from the vesicle, making their way down through palisade parenchyma to spongy tissues, but never inserting haustoria into the palisade cells, or very rarely, if any. On the other hand, the development of the fungus is mostly achieved in the spongy tissue, producing many haustoria until the time of teliospore-formation which usually takes place after about 4-5 weeks. In any cases studied noncompatibility between host and parasite can not be clearly demonstrated.(b) The plastids of the palisade cells in close contact with the invading hyphae as well as those of several cells near the hyphae first reduce in size and function, but after the actual invasion of haustoria these degenerated plastids recover in size, and also seem to become more active in function than ever. In the case of the spongy tissue, the plastids of the cells become larger and more greenish in color, and increase in number when actually invaded by the haustoria. Moreover, starch grains are found more numerous in the invaded cells. The starch grains present in the invaded palisade and sponge cells are different in form, viz., simple (fig. 10) and compound (fig. 11), respectively.(c) The affected cells are not hyperplasied in any noticeable degree, but more or less hypertrophied (fig. 8).(d) The haustorium has a slender neck at its base, and generally small and simple, though some digitate haustoria (fig. 13, 11) are usually found in the cells surrounding the substomatal cavities. The haustorium is as a rule incased in a thick wall and dead when old (fig. 15, 16).(2) Immune region, viz., very young, rapidly growing portion of the phyllode.In this developing stage of phyllodes stomata have not yet been sufficiently developed, so that the parasite is unable to enter the host tissue except in the case of more advanced stage, in which the parasite is able to enter through stomatal apertures, though with much difficulty. The entering hypha produces the substomatal vesicle in the same manner as stated above, but the parasite can not further develop at all and is soon perished (fig. 5, 6) as a result of the toxic secretion from the host cells (cf. 8). In rare cases, however, the fungus can develop in a few days and produce some rudimentary haustoria, but it never continues to grow any further and is finally perished (fig. 18) prior to the death of the host cells, though there are found several cases where it is impossible to determine which is first deceased. On the other hand, the guard cells beneath the appressoria and the epidermal cells connecting with the former are very frequently killed (fig. 4) though this occurs less frequently in the advanced stage of host development, it is presumed that the parasite may also secrete some toxic substance, to which the host cells are more sensitive when they are younger.(3) Immune region, viz., mature, fully grown portion of the phyllode.The parasite is able to enter the host through stomatal apertures with ease, and produces the substomatal vesicle as usual, provided that the urediospores can germinate and produce appressoria (in nature, as has already been stated in a previous paper, the urediospores can hardly germinate, or none, on such a portion).