WHEAT-RUST.

New Zealand Journal of Agriculture, Volume XIII, Issue 1, 20 July 1916, Page 41

WHEAT-RUST.

A NEW ZEALAND OBSERVATION

R. WATERS,

Biological Assistant

The occurrence of wheat-rust [Puccinia graminis) in New Zealand is by no means uncommon ; . in fact, its presence may be detected more or less every season. The severity with which it attacks and the ", extent .of country that it seriously affects is, however, often so limited that farmers either regard its effects as trivial or even fail to note its . presence at all. Under conditions favourable to its development wheat-rust occasionally assumes the .form of an epidemic in which a large acreage in wheat-growing , areas may be v seriously affected. Such . was the case . during the past season in the vicinity of Greenfield, Bruce ■ County, South Otago. There the yields over a wide area were reduced from 30 down to about 10 bushels per acre, while several hundred . acres of crop had to be merely gathered up and burnt, the, grain being so shrivelled as to be unmarketable. , .

CAUSE OF THE DISEASE.

An investigation of a large portion of the affected area, made at the instance of the Greenfield Farmers' Union, showed that while a certain amount of damage may have been caused by

frost, yet in the main the wheat crops had suffered from the effects of rust. Puccinia graminis is a form of plant-life which is classed with the ordinary field mushroom as a fungus. Like the mushroom, it is unable to derive its sustenance direct from the soil. While the mushroom is large in size and lives upon decaying animal and vegetable matter deposited in the soil, Puccinia is so small as to be seen in detail only with the aid of a microscope. It nourishes itself upon the food substances on their way from the roots to the seed-heads of wheat-plants. This misappropriation by

the fungus of the food materials intended for the development of wheat-grains readily accounts for the undeveloped and shrunken appearance of the grains of affected plants.

To the naked eye one of the first signs that a crop is affected is the appearance on the stems of a fine reddish powder which comes off and sticks to the clothes or blows about in the wind. Under the microscope this reddish powder is seen to consist of innumerable red globes borne in very narrow slits or crack-like ruptures (sori) in the skin or epidermis of wheat stems and leaves.

These slits may be seen with the naked eye if examined closely. They arise in the following manner: The z fungus having penetrated the outer skin (epidermis) of wheat-plants, then develops minute root-like fibres (hyphae), which ramify amongst . the inner tissues of the plant. These “ roots ” draw upon the plant-juices, increase in length and number, and finally burst through the outer skin of the stem's, leaves, and bracts, forming slits therein. Here they produce abundantly those red globes which are the so-called summer spores (uredospores) 'of the fungus that is, the reproductive bodies by which wheat-rust spreads in the summer from one wheat-plant to , another and from one crop to another. Highly magnified, their appearance is similar to Fig: 1 (a). The name “ wheat-rust ” is given to the disease on account of. the rusty-red appearance lent to affected crops by these spores, particularly in the summer months. Later on there arises from the same slits a black powder— so-called winter . spores (teleutospores) of ,the fungus. Fig. 1 (5) shows two winter spores highly magnified. The presence of this black-spore form in the slit-like ruptures was greatly in evidence at the time the investigation was made (end of March last).

Both the summer and the winter spores are the reproductive bodies of the same fungus Puccinia graminis. The especial function of. the summer spores is to propagate the fungus in the summer, while that of the winter spores is . (r) to withstand severe winter conditions;' (2) to , germinate the following spring and themselves produce another special kind of spore known as the basidiospore (Fig. 1 (&)). The function of the basidiospore is to infect the barberry (Berberis vulgaris) as a -plant. The fungus resulting from the basidiospore . creates a fungus disease of the barberry, and ultimately produces a spore-form distinct from any of the . three previously mentioned namely, the acidiospore. The acidiospores are dispersed by the wind and infect wheat-plants with Puccinia graminis, which fungus then proceeds to produce, the summer spores, and so on as before.

In New Zealand the occurrence of barberry is so rare in many parts that it would appear impossible for it to. bear the fungus producing the acidiospores which originate the wheat-rust ; moreover, so far the occurrence of affected barberry shrubs has not been recorded in New Zealand. The usual means by which wheat-rust survives during the winter is therefore apparently not resorted to in New Zealand, and the question as to how it winters here is still obscure. The summer spores, which are believed to perish in countries with hard winters, are, it is asserted, capable of maintaining their , viability in certain countries with

milder winters, and of . carrying the disease over from season to season.' The most satisfactory explanation at present of the means by which the fungus continues from season to season in this country is that the summer spores infect the aftermath of a . wheat crop (the self-sown plants from a previous crop) and autumnsown crops, and thereon again produce summer spores, which later disperse and infect the spring -sown crops. As will be seen later, there is strong evidence of the Greenfield outbreak having spread more especially from an . autumn-sown crop. .

THE RELATION OF WEATHER TO DEVELOPMENT OF THE FUNGUS.

. . While the ’ spread of the disease in subsequent seasons is dependent upon the presence of spores, the extent to which they can infect and spread on future crops depends upon certain conditions, the chief of which undoubtedly is the weather. . The exact temperature and degree of humidity favourable to the development of wheat-rust is unknown, but in general. the moist, steaming, and hot conditions, similar to those of a hothouse, seem to favour, the germination of the spores, their subsequent penetration of the epidermis, and the growth of the root-like fungus fibres amongst the tissues of the wheat-plant. Thus foggy • weather seems to favour the fungus; also hot weather, especially following upon dewy nights; and also overheated soil (caused by a previous drought) with showers that cool the ground but little and themselves are largely given off in vapour. It will be seen that the presence of the rust-spores alone in- a locality does not necessarily result in an outbreak, which is possible only when suitable atmospheric conditions prevail for a sufficient length of time. So also the affected districts of this, season may or may not suffer materially next year, according to the extent to which the spores are present and the character of the weather next season. Hard winters, no doubt, are very important means by which the enormous numbers of the spores of wheat-rust are kept within bounds.

SUBORDINATE FACTORS AFFECTING THE YIELD OF WHEAT.

The effect of rust in the Greenfield district upon the yields per acre of grain varied considerably. In some cases an affected crop yielded over 40 bushels, others not more than 25. Again, there were crops that would return only 10 or 12, and a number that gave nothing at all. The reasons for this were carefully inquired into, and may be stated as follows :

(i.) The atmospheric conditions certainly varied somewhat in the Greenfield district, rendering one part less favourable to an outbreak than another. Some parts were more subject to fogs

than others, and some held the rain that fell, while others lost it through natural drainage. The damper soils would be cooler, the constant but slower surface evaporation resulting in reduction of temperature, whilst the drier soils would be hotter and would disperse volumes of moisture as vapour after a . shower. The superabundance of . water - vapour in the atmosphere after the heavy thunder - showers just prior to the outbreak is one of the suspected contributory causes of the trouble, the hot sunshine about the same time being a necessary accompaniment.

(2.) The soil noticeably varied in different parts, some being reputed as much better wheat lands than others. The constitution of wheat-plants' would undoubtedly vary according to the suitability of the land for this crop, and their susceptibility to damage by rust - infection would consequently be greater or less according ■ to their weakness or vigour, their backwardness or precocity.

(3.) Rotations play an important part. Land out of lea is well recognized as inferior for wheat to land previously carrying turnips. Crops whose development was advanced by good rotations had produced better - filled grains by the time the fungus arrested their progress.

. (4.) The date of sowing certainly seemed to have been directly connected with the amount of the yield of affected crops. Quite a number of ' instances were forthcoming: Early-sown crops gave comparatively good yields. Some growers who sowed in July or early in August got over 40 bushels from a rust-affected crop. Later sowings in general gave relatively smaller yields, while some who sowed in ' September found it quite unprofitable to thresh.

The explanation is very apparent. The outbreak in this district was very, sudden. The fungus seized upon the stems and leaves of the wheat, intercepting the flow of plant-juices to the grain. This (with perhaps the exception of a certain crop sown in the autumn) happened practically simultaneously throughout the district, and the early-sown crops, having greater grain-development than the late-sown ones, were consequently able to ripen a better yield. The late - sown crops, being more milky when attacked, correspondingly produced a greater percentage of poorly filled or much-shrunken grains. . 1

CENTRE OF INFECTION

Throughout the district were one or two "autumn-sown crops which, had they been affected at about the same time as those sown in the spring, and had other conditions been equal, should have yielded better than any of the spring-sown crops —the grain development of the former, theoretically, being greater than that

of the latter at the time of ' the attack. Strangely enough, however, one of these autumn crops in the affected area, instead of yielding comparatively well,' had produced so poor a yield. that it had been decided to burn off the lot. Inquiries elicited the fact, moreover, that this crop had not been attacked at the same time as the others — presence of the rust having been reported upon it some time before the outbreak became general. The land upon which this crop was grown had two seasons before been occupied by a rust-affected crop,- seed from which was used this season. While the outbreak is not considered as due to the use .of seed from a rust-affected crop, yet it does seem very likely that this particular autumn crop became very early affected by spores emanating from self-sown plants from the crop of two seasons ago, and that the centre of infection for the remainder of the district in the season now dealt with was this early affected autumn-sown crop. - .

METHODS OF TREATMENT.

As already shown, it is advisable for the control of the rust to destroy quickly, as far as practicable, all the aftermath of wheat crops and self-sown wheat. The sowing of autumn crops is probably inadvisable, and early spring sowing is undoubtedly. good. In the application of fertilizers it is well to remember that excessive nitrogen delays the early maturing of the grain, whilst phosphate fosters it and thus works to the end of getting as much grain-development before the rust appears. Any cultural or other measure which makes for early maturity and vigour is of value.

The so-called rust-proof varieties of other countries or places are more correctly -resistant; moreover, they are not necessarily even rust-resistant out of the environment in which they have ' been raised. It follows, therefore, that each rust-resistant variety will best be evolved in the district where it is eventually destined to be grown for a crop.

There is no satisfactory treatment known for a crop already affected, and the methods of preventing attack are at present still far from satisfactory.

PUCCINIA GRAMINIS AND OATS.

It is worthy of note that the odd oat-plants found here and there in some of the badly rusted wheatfields in the Greenfield district had practically no signs of rust, and had developed wellfilled grains.