Wheat Rust

Of all the diseases of wheat, and there are many, rust is by far the worst. All plant rusts are caused by the infection of a plant by a Basidiomycete fungus. Most Basidiomycetes are fleshy fungi that produce mushrooms, conks, and puffballs and are either saprophytes or wood decaying organisms. But the Basisiomycetes also included two destructive groups of plant pathogenic fungi, the rusts being one of them. Wheat plants are susceptible to three different rusts, stem rust (Puccinia graminis), leaf rust (Puccinia recondita), and stripe rust (Puccinia striiformis). Of these, the most serious one is stem rust. Stem rust affects wheat where ever it is grown, decreasing yields and in bad years even killing the plants. Bad rust years in North America, for example, can cause losses of tens to hundreds of million metric tons of wheat.

The stem rust fungus attacks the stem, the leaves, and the tissue around the seed heads of the wheat plant. It causes blisters that eventually rupture the epidermis, exposing a powdery mass of rusty red-colored spores, hence the name. These spores, called uredospores, can reinfect the same wheat plant or infect other wheat plants. The uredospores are easily picked up by wind currents and blown hundreds of miles away from their original source. If a uredospore lands on wheat plant, it will germinate in the presence of moisture and send a hyphal strand, called a germ tube, into a stomata. The fungal hyphae ramify through the plant tissue, penetrating mesophyll cells and draining them of their soluble foods. Within 8-10 days after infection, the fungus makes spore-producing structures that rupture the epidermis and produce more uredospores.

The importance of the East Wind to the rust fungi in Ancient Egypt was two-fold. First the winds pick up the fungal spores from infected plants and deposit them in the wheat fields of Egypt. Second the winds are followed by heavy rain clouds that sweep over the coast into the Eastern Mediterranean providing rain and thus the moisture for spore germination. Wheat rust is always much worse in moist or wet weather because more spores germinate and infect the wheat plants. The pharaoh did not stand a chance. With little understanding of the causes of disease, there was nothing he could do to preserve maat in the face of “blasting” East Wind.

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