15-20 bushels (900-1,200 lb.) per acre. In exceptional circum stances small areas of a few acres in extent have given up to Ioo bushels (6,000 lb.) per acre.
In early summer are seen narrow elongated rust coloured pustules on the stems and leaves; these are the sori of the rust fungus, Puccinia graminis, from which are shed vast numbers of reddish-yellow uredospores. When placed on the leaf or stem of the wheat plant the spores germinate and penetrate its tissues giving rise in a short time to another sorus ; in this manner the disease is spread rapidly through the crop during summer. Later in the season the sori are almost black, producing dark-brown teleutospores which do not germinate until the following spring, when they give rise to small secondary spores ; these cannot in fect wheat but readily attack the leaves of the barberry (Ber beris vulgaris). Within the leaves of the new host the fungus grows and bears aecidia or cluster-cups, which break through the epidermis and shed large numbers of aecidiospores ; the latter are unable to re-infect the barberry, but when carried to a wheat plant germinate and penetrate the young leaves producing there an internal mycelium from which uredospores like those first mentioned are given off.
Several biological species of the black stem rust fungus are known, which though similar in morphological characters are different in their parasitism. One biological species attacks wheat, barley, rye and several wild grasses ; another is parasitic only on oats and certain grasses, a third lives upon rye and barley and will not infect wheat or oats, while three or four are only capable of using certain grasses as hosts.
In districts or countries from which barberry is absent suc cessive crops of wheat are often very seriously infected with rust ; the explanation usually given for the prevalence of the disease under these circumstances is that the parasite persists from one season to another in the uredo stage upon stray wheat plants or wild grasses. There is still, however, considerable ob scurity regarding the origin of attacks of black stem rust in barberry-free regions.
No specific remedies for rust diseases are known, but black stem rust is less prevalent in areas from which the barberry host has been eradicated. Varieties of wheat differ considerably in their susceptibility to these parasites, and it is hoped that by selection of immune kinds or the production of resistant hybrids, the damage to crops may be checked or largely reduced.
Another disease of serious import to the wheat grower is bunt or stinking smut, Tilletia tritici. Bunted grains produced by dis eased plants have a disagreeable odour of decaying fish, and are filled with a mass of black spores which are set free during the threshing process, becoming attached to the coats of the healthy grains. Germination of the bunt spores and the contaminated grains occur at the same time when sown, and the fungus infects the young plant soon after it makes its exit from the grain. The parasite lives in the wheat plant without seriously affecting its growth, progressing upwards in the stem and finally entering the developing grain where the mycelium of the fungus is transformed into a mass of bunt spores. The disease can be checked by steep ing the seed grain in solutions of copper sulphate, formalin or other fungicides which destroy the spores on the contaminated grains without damaging the embryos within them.
Allied to bunt is common smut of wheat, Ustilago tritici, a parasite having a life-history similar to that of bunt, but it gains an entrance into its host through the flowers.
For further knowledge consult The Wheat Plant: a Monograph, by J. Percival (Duckworth and Co., 1921). (J. P.) Great Britain.—In 1929 wheat occupied only about one seventh of the arable acreage in England and Wales as a whole and little more than one-twentieth of the total cultivated area. These proportions are somewhat lower than those which obtained in the decade immediately preceding the World War; and while the proportion of the arable land devoted to this crop has become almost constant—varying mainly with soil and weather conditions at the time of sowing—there is still an annual shrinkage of arable acreage, and with it a reduction in the wheat crop.