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Pathogen Biology

Tilletia tritici, T. laevis

The genus Tilletia was named by the Tulasne brothers in 1847 to honor M. M. Tillet who in 1755 worked with this pathogen in wheat. Since then two species of Tilletia have been shown to be involved in this disease—T. tritici (syn. T. caries) and T. laevis (syn. T. foetida). The two species are distinguished based on the morphology of the bunt spores (Figures 6a and 6b) or by modern molecular techniques, i.e., repetitive sequence based polymerase chain reaction (PCR). Teliospores of T. tritici have ridges on their walls which gives them a “warty” appearance while teliospores of T. laevis have smooth spore walls. Tilletia tritici is found in the drier western areas of the United States whereas T. laevis occurs in the wetter areas of the mid-west and eastern United States.


Figures 6a	Figures 6b	Figures 7

Reproduction

The two Tilletia species have the same life cycle. Dark-colored teliospores form in bunted kernels and are the overwintering structures. They are released from infected plants (see Disease Cycle and Epidemiology) and land on healthy seeds or on the soil surface. Karyogamy and meiosis occur in the teliospore. When environmental conditions are favorable, each teliospore germinates by producing a short germ tube, technically called a basidium. Shortly after the basidium is formed, long thread-like, light-colored, haploid spores are produced, four or more per basidium (Figure 7). These are called primary sporidia or basidiospores. Genetically compatible sporidia anastomose (fuse) very quickly to form an H-shaped structure (Figure 7), which allows the haploid nuclei in the two sporidia to come together as a dikaryon. The two nuclei do not fuse together, but function as a pair. An infectious hypha develops from the H-structure. This dikaryotic hypha penetrates into the tissue of a germinating seedling and establishes itself just behind the growing point or apical meristem of the wheat plant.

Infection of wheat by the common bunt pathogen represents a race in which the fungus attempts to reach and establish itself in the developing apical meristem of each tiller before internode extension rapidly moves the apex upwards beyond the reach of the fungus. If environmental conditions favor the pathogen, infection and a consequential bunting of the majority of spikes of a susceptible cultivar will occur; conversely, if conditions are not optimal or if plant defense responses impede or stop pathogen ingress, infection will fail resulting in few or no spikes being infected. Just when the wheat head is formed, the hyphae of the smut fungus invade the newly developed seed and begin proliferating rapidly. The hyphae replace the cells of the seed, so that finally only the seed coat remains. Individual cells of the smut fungus then “round up” to form the black, smelly teliospores. The two nuclei eventually fuse to form a diploid nucleus in the teliospore.

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