YELLOW-LEAF DISEASE IN PHORMIUM TENAX.

New Zealand Journal of Agriculture, Volume XXIV, Issue 1, 20 January 1922, Page 27

YELLOW-LEAF DISEASE IN PHORMIUM TENAX.

PRELIMINARY REPORT ON A CURRENT INVESTIGATION.

R. WATERS and E. H. ATKINSON,

Biological Laboratory, Wellington.

INTRODUCTION.

Where the disease under consideration is rife and the foliage of -many acres is abundantly yellowed the commonly adopted name of “ yellowleaf ” is truly descriptive of the general appearance. It is not especially the shade of yellow nor any other specific quality that has le I to the adoption of this term, but. rather the predominance or generality of the colour over areas where the disease is bad. We have shown, however, that lack of water or even an excess of it may be followed by yellowing in pho-leaves, as also may too little light. The yellowing of the foliage, therefore, like pain' in the human body, 'is a very general symptom, and one . that may arise from a variety of causes. Nor yet does the order in which the leaves discolour constitute a distinctive feature of yellow-leaf, for the same order is maintained when yellowing is produced by various means.

That certain large areas of phormium have yellowed and died rapidly there is no doubt, and that such yellowing differs, at least in its rapidity, from what is usual in healthy flax is equally apparent ; yet as a symptom of the diseaseat one time the only known symptom the yellowing of . the leaves was found, to be a character of little or no value for the purposes of specific identification. When, therefore, it was asked, “ What is the cause of yellow-leaf ? ” one was forced to inquire, “ What specifically is the, disease referred to ? ” And it became clear that until a more definite idea could be’ formed as to the symptoms distinguishing yellow-leaf ” from various other, yellowing processes little advance could be made towards arriving at 'the specific cause of the. disease. In this particular disease, therefore, the symptomatology presents as much difficulty as the etiology.

FUNGUS IN THE ROOTS.

Our first problem, therefore, was further to define the disease by associating it with' other symptoms more distinctive than the single one that we namely, the orange-yellowing of the foliage, starting with the outermost leaves and progressing inwards towards the centre of the fan, successively discolouring such leaves often at a rate greater than that at which they can be replaced at the centre. • ' .' .

It is a very obvious fact that badly affected bushes are remarkably easily uprooted, and that the reason for this is the rotting of certain of the roots. In studying the pathology of yellow-leaf one is therefore led at once to observe the coarse microscopic fungus in the tissues of the roots, more especially in the finer roots.- This' would certainly constitute a very distinctive symptom if it proved to be causative. In the attempt to prove or disprove . the connection of this fungus with the disease the-following facts-have been ascertained : —

(i.) A large number of microscopic sections prepared by Mr. G. H. Cunningham, of this Laboratory, showed that there ; was no trace of the fungus in the seed. ' ■ * • ■■ (2.) No trace of fungus was found in the roots of eleven-month-old seedlings raised from corrosive-sublimate-sterilized seeds in sterilized river-sand. These two first observations indicate that the fungus is not present in the tissues of the seed.

(3.) No trace of the fungus was found in the roots of eleven-month seedlings raised from unsterilized seeds in sterilized river-sand. This would indicate that the fungus is not in general carried over upon the exterior of the seed. .

■ - (4.) The coarse vacuolated fungus-strands ■ were, however, found abundantly in the root-tissues of seedlings of the same age raised from the same collection of seed, but sown in soil from a flax-swamp where yellowdeaf was abundant. This indicates that the fungus is maintained in the soil, whence it invades the roots of flax. ' : ' ■ ■

(5.) The fungus was found in the roots of year-old seedlings raised in soil- from a swamp free of yellow-leaf disease, in ' nine-month seedlings raised in Wellington garden soil, and in nine-month seedlings nursery-raised. This shows that the fungus is present in various soils, and not exclusively where yellow-leaf disease is met with.

(6.) Typical coarse fungus-strands are common in the roots of adult yellow-leaved phormium, but they are equally common in the roots of bushes showing no signs of this disease.. (7.) Such strands have been observed in roots taken from isolated phormium patches apparently perfectly . free of the disease and miles distant from any affected phormium. ; (8.) The fungus is commonly found in the roots of Phormium Colensoi as well as P. ten ax.

(9.) Fungus-strands in phormium-roots' have been found in material from Dunedin, Seatoun, Kelburn, Karori, Plimmerton, Waikanae, Levin, Shannon, Makerua, Tokomaru, and Paparoa, the latter place eighty-five miles north of Auckland. Thus the fungus is found in healthy and diseased bushes alike, in both phormium species, and is widely distributed throughout New Zealand. ■ (10.) The fruit (chlamydospores) of the . fungus was rarely observed in the root-tissues.

(it.) The fungus has been isolated from phormium - roots and successfully grown on artificial culture media. (12.) It has fruited freely (producing both chlamydospores and conidiospores) oh artificial culture media. (13.) Pure cultures sent to Kew Botanic Gardens, England, were identified as a species of Ramularia.

(14.) Certain Ramularia species are already recorded as parasitic on the leaves, stems, and roots of plants other than phormium.

(15.) Pure' cultures have been inoculated into the roots of healthy phormium seedlings, and in several cases lesions were found about the point of inoculation.

(16.) Typical yellow-leaf, however, has not yet made its appearance in the foliage of any of the inoculated plants. Thus Koch’s postulates have not been fulfilled. We have certainly been obliged to modify our views with regard to Ramularia. We hold that it is a saprophyte and a parasite. Its parasitism is frequently not markedly destructive -—it may never be— there is a possibility that under some conditions it is. While, therefore, the presence of this fungus may not be regarded as a symptom of yellow-leaf, there is still the possibility that suitable . conditions may serve to convert it into a parasite more destructive than it appeared in the roots of inoculated plants, and sufficiently destructive to result in the yellowing of the leaves. Experiments are now in progress to show any relative parasitism that may be exhibited by the fungus under various environmental conditions.

TESTS FOR MICRO-ORGANISMS GENERALLY.

Seeing that the Ramularia inoculations might take many months to demonstrate pathogenicity, it was decided in the interim to set out on an entirely different course. Reviewing the whole position it was clear that none of the previous investigators of this disease had put forward any incontrovertible or even convincing evidence that yellowleaf was caused by some micro-organism, nor were we in a position to assert that it was. It appeared that a series of carefully devised experiments might reasonably be expected to yield some facts as to whether the disease was due to a living organism or an association of organisms. We shall not attempt more than briefly to sketch the main lines that were pursued with this purpose in view.

In these experiments it was necessary to raise a large number of seedlings; the commercial seed available was found to be quite worthless. Some useful data were. secured concerning the saving of phormium-seed and the propagation of seedlings, a matter in which there has been some difficulty in oversea countries desirous of establishing the plant. The following experiments were then established in duplicate or triplicate —

First Series.

(i.) Seed collected from what appeared to be yellow-leaf “resistant plants” was sown in phormium-swamp soil where no yellow-leaf could be found (hereafter referred to as “ uninfected bog soil ”).

(2.) Seed collected from yellow-leaf plants (temporarily regarded as “ susceptible seed ”) was likewise sown in uninfected bog soil.

Note.— should here be recorded that in the flowering and seeding time one of the most distinctive and constant symptoms associated with the yellow-leaf condition was found to be the failure of the affected plant to put up a flower-stalk ; even if it managed to do so the stalk was frequently stunted, and was rarely able to bring more than a few seeds to maturity.

(3.) Resistant and susceptible seed sown in alternate rows in uninfected bog soil.

(4.) Resistant seed sown in soil collected from beneath a number of badly yellow-leaved plants (“ naturally infected bog soil ”).

(5.) Susceptible seed sown in naturally infected bog soil. (6.) Resistant and susceptible seed sown in alternate rows in naturally infected bog soil.

This series of experiments aimed at showing whether or not the infecting agent was to be sought in the soil, and, if so, whether there was any relative resistance or immunity in the seeds. The seed was sown in boxes at two different times of the year, so that very delicate roots would be available to any infecting agent for a long time and under considerably varying ranges of temperature and humidity.

Second Series.

(1.) Phormium seedlings (a little under twelve months old) planted in pots of river-sand sterilized at 160° C. for two hours.

(2.) Phormium seedlings planted in sterilized river-sand which was subsequently inoculated with Ramularia.

(3.) Phormium seedlings planted in naturally infected bog soil.

This series aimed at showing various points, amongst which were—(a) Whether the infecting agent was a micro-organism associated with the seed; (&) whether it was Ramularia ; (c) whether it was contained in the soil.

Third Series.

(i.) Phormium seedlings planted in pots of garden soil (referred to as ‘‘uninfected garden soil”). (2.) Phormium seedlings planted in uninfected garden soil in which was buried diseased roots collected from typically yellow-leaved bushes. (3.) Phormium seedlings planted in naturally infected bog soil. (4.) Phormium seedlings planted in uninfected garden soil subsequently inoculated with Ramularia.

This series was an attempt to show whether the infecting agent was to be sought in the diseased roots of typically yellow-leaved phormium, whether it was located in the soil, and whether it was Ramularia.

Results.

These three series were instituted nearly twelve months ago. Unlike positive results, negative ones are by nature somewhat inconclusive. Such. are the results obtained from these experiments to date. The following are some of the possible, interpretations of these results : —

(i.) Seeing that no trace of the yellow-leaf condition has yet appeared above ground in any of the three series, the micro-organism —if such be the causewould appear very exacting in its environmental requirements or but a weakly parasite.

(2.) Or such micro-organism may not have found in the conditions provided in the experiments such an environment as would enable it to assume destructive parasitism. (Other experiments have since been instituted to secure more light upon requisite environmental conditions.) (3.) Again, seeing that Ramularia is so widely distributed throughout New Zealand and that it failed to result in the yellowing of these seedlings, the indications are that it is not causative.

(4.) On the same lines the lack of positive results in all three series gives us no reason to believe that yellow-leaf is caused by any microorganism whatever; but, while these experiments are fairly extensive ones, it would at present be quite unsafe to accept this negative evidence as fact. There is much work to be done before such a conclusion should be entertained.

RELATION OF INDIVIDUAL ROOTS TO INDIVIDUAL LEAVES.

The suggestions were made that the death of the older roots was resulting in the yellowing of the older, leaves ; that the trouble might be physiological : and that the disease might consist largely in a mere acceleration . of what normally took place. This led to the following experiment to prove whether or not certain roots fed certain leaves : Certain main roots were cut and were instantly fed with coloured water. Red, which was supplied through only one root, made its appearance in the vascular system of nearly every leaf of the fan. The colour green simultaneously fed through another single root appeared in many leaves, and, moreover, in some of those into which red had also made its way from an entirely different root. This experiment shows that individual roots serve many leaves. Other experiments have shown that damage from adverse conditions of various kinds commonly is shown in the outer leaves first. A. deficiency of functioning roots, as in transplanting, results in the death of the outermost leaves.

BACTERIA IN RELATION TO YELLOW-LEAF.

A bacteriological study was then commenced of the finer roots which on affected phormium were very commonly seen to be discoloured and rotting. Six different organisms were isolated ; they were immediately grown separately in quantity, and then were mixed and applied to' pots of phormium seedlings to see whether one or an association of them could set up the disease. The pots were incubated at 25 0 C. for twenty-four hours, thereafter kept in the laboratory a few days, and finally sunk in the ground outside. In twelve months the seedlings have shown no signs of disease above ground. Other organisms, both bacteria and fungi, have from time to time been similarly tested, but with negative results.

RHIZOME-CANKER.

Some time ago it was noted that parts of the vascular system of the rhizome of affected phormium occasionally showed discoloration, and some of our later expeditions to the swamps were devoted mainly to this matter. A considerable amount of material was dug up and sectioned on the spot, the result being the discovery of a cankerous condition of the rhizome associated very commonly indeed with plants in the advanced stages of yellow-leaf. The flora of this canker is now being studied. A short round-ended diplo-bacillus has been isolated and inoculated into phormium seedlings. The inoculated plants have been subjected to saturated soil conditions and relatively high temperatures such as occur in the swamps. A number of phormium seedlings have, moreover, been recently planted out in a swamp in places where the disease is rife. Again, cylindrical plugs from the cankered rhizomes of yellow-leaved bushes have been cut out with a cork-borer, and have been inserted into holes made in the rhizomes of a healthy phormium plant, suitable controls being set up. It is, however, too early to expect any results from this work.

CONCLUSION.

In concluding these notes we would like to make plain the following points : It should be clearly recognized that while every endeavour is being made to trace the cause of yellow-leaf disease to a specific organism, nevertheless in the light of our present knowledge it cannot be assumed that a micro-organism is certainly the cause. Even if the trouble is finally traced to a specific organism the present evidence would show that only under precise conditions does it exhibit destructive parasitism. We would therefore stress the study of requisite environmental conditions ; it may ultimately prove more productive of control measures than a knowledge of the causal agent.