REPORT OF THE ESTOMOLOGIsT Ayo BOTAMST 253 



SESSIONAL PAPER No. 16 



THE EFFECT UPON THE WHEAT PLANT. 



The physiological effect upon the wheat plant by the presence of the rust parasite 

 is better understood by a consideration of the life history of the minute plants which 

 are known as rusts. The term JRust, as applied to cereals, describes a disease due to 

 the attacks of several different parasitic fungi belonging to the Uredinese, a family 

 which includes the most destructive parasites of cultivated and wild plants; and it 

 must not be forgotten that rust is a plant, and, although so minute that a strong 

 microscope is required to examine it, it is just as much a true plant with a definite 

 life history of its own, as the wheat, oats, grasses, &c,, upon which it grows. 



The general belief that rust comes with rain, fog, or heavy dew after a hot day, 

 is in the main correct; but the moisture and hot air are not actually the cause of the 

 trouble; they merely act as the carriers of it and provide the conditions necessary for 

 its injurious propagation. 



The rust which was answerable for nearly all the injury in the West last season, 

 was the Black Stem Rust. There are about a dozen different kinds of rusts which 

 occur on wheat, oats and barley in this country. The commonest of these are the 

 Orange Leaf Rust (Puccinia rubigo-vera) or Spring Rust, and the Black Stem Rust 

 (Puccinia graminis), or Summer Rust, which attack all kinds of small grains, and the 

 Crown Rust, or Orange Leaf Rust, of oats (Puccinia coronata) , which does not occur 

 on wheat or barley. Each of the first two named species has distinct specialized forma 

 which attack wheat, oats and barley and some other grasses, but which very seldom 

 infest plants belonging to other grains than those upon which they developed. For 

 instance, spores of the Black Stem Rust of wheat will not produce readily on either 

 barley or oats the corresponding rusts of those plants and vice versa. The two common 

 rusts of wheat occur in all parts of the world, where that staple crop is grown; and 

 in almost every instance it has been found that the Black Stem Rust is by far the 

 more injurious of the two. The Orange Leaf Rust appears earlier in the season and 

 is the more conspicuous; but the later-developed Black Stem Rust attacks its host 

 in a much more vulnerable spot, namely on the stem, the channel up which the nutri- 

 tious principles are carried from the vegetative system of the plant to be stored up 

 in the seed. Developing on the stem, it arrests and feeds upon these important ele- 

 ments, thus causing starved and shrunken grain. The Orange Leaf Rust of oats is a 

 different species from the Orange Leaf Rusts that occur on the other small grains ; and 

 like them has a red rurit or spring form and a dark-coloured or summer form; but the 

 Black Stem Rust of oats is merely a specialized form of the species {Puccinia gram- 

 inis), which is also found on wheat, barley and rye, as well as on many different kinds 

 of grasses. 



THE GROWTH OF THE PARASITE. 



In the case of the Black Stem Rust, the growth of the parasite is the same, what- 

 ever its host plant may be. It passes the winter in a resting condition on tftie old 

 stems of the previous year. In the fields this will be chiefly on the stubble. The winter- 

 spores or seed-bodies germinate early in spring and produce another kind of spores, 

 which are exceedingly light, and are borne from place to place by the faintest breath 

 of wind. These, alighting on the growing grain plants, produce, later, what is known 

 as the red-rust or uredo stage of the fungus, to be followed in autTunn by the resting 

 winter-spores of Black Stem Rust. The sequence of this development is as follows: 

 As soon as the minute spores of the first germination are carried on to a leaf of a 

 growing plant, they germinate and throw out very slender tubes, which enter the 

 tissues of the host plant in the same way that roots penetrate the soil. Here they feed 

 at the expense of their host, and in time produce large numbers of reddish brown 

 spores, which burst through the tissues and cause the red-rust stage, which again, 

 later on in the season, is followed by the black-rust stage, which consists of the pro- 



