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Pathogen Biology

The fungus that causes rice blast is called Magnaporthe oryzae (formerly Magnaporthe grisea) (Figure 9). It is an ascomycete because it produces sexual spores (ascospores) in structures called asci, and is classified in the newly erected family Magnaporthaceae. The asci are found within specialized structures called perithecia. The mycelium of M. oryzae is septate and the nuclei within the mycelium and spores of this fungus are haploid.

Sexual Reproduction
The sexual, or teleomorphic, stage of the rice blast pathogen can be produced in the laboratory if isolates of opposite mating type are paired, but has not been found in the field in the United States. As an ascomycete, it produces hyaline, fusiform shaped (spindle-shaped with tapering ends) ascospores with three septa. The asci are unitunicate. This fungus is considered to be heterothallic with a bipolar mating system (mating controlled by two different alleles at a single locus) with additional genes controlling the sexual cycle. Based on recent phylogenetic, molecular and morphological data, isolates of the fungus from rice and closely related isolates from other grasses like Eragrostris curvula, Eleusine coracana, Lolium perenne, and Setaria spp. are taxonomically described as Magnaporthe oryzae, while isolates from Digitaria sanguinalis (crabgrass) are distinct and should be described as Magnaporthe grisea.

Asexual Reproduction
The asexual stage of Magnaporthe oryzae is described by the name Pyricularia oryzae (formerly called P. grisea) and it is the most common spore form of the fungus in the USA (Figure 10). These spores, called conidia, are produced abundantly on lesions and in culture on specialized stalks, called conidiophores. The conidia are usually three-celled, and produced on the apex of a conidiophore. Sporulating colonies on agar plates may take on a fleecy grayish appearance.

Under favorable conditions, the fungus sporulates in the center of the lesions on susceptible cultivars (Figure 11). It can also sporulate on seed lesions (Figure 12). It rarely sporulates on the most resistant cultivars. Spores are produced on infected leaf, collar, panicle, and seed, on conidiophores that extend beyond lesion surfaces; the conidiophores and spores en-masse may give the lesions a dusty gray appearance. Conidia are produced after several hours of high humidity and are easily released or liberated near mid-day, especially under windy conditions.

Figure 9	Figure 10
Figure 11	Figure 12

The Infection Process
Infection of rice occurs when conidia are deposited on rice tissues, and germinate by producing a germ tube and an appressorium. The appressorium is a melanized structure, and from it develops an infection peg which penetrates the tissue. After penetration, the primary infection hypha grows rapidly and ramifies within susceptible tissues. Growth within tissues of resistant cultivars is often inhibited.

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