D. R. L. Steindl TESTING SUGAR CANE VARIETIES FOR DISEASE RESISTANCE AT THE BUREAU PATHOLOGY FARM, QUEE~~SLAND

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1 TESTING SUGAR CANE VARIETIES FOR DISEASE RESISTANCE AT THE BUREAU PATHOLOGY FARM, QUEE~~SLAND D. R. L. Steindl Senior Pathologist, Bureau of Sugar Experiment Stations, Brisbane, Queensland, Australia The testing of all new varieties for their reaction to the more serious diseases which occur in Queensland or which have occurred here in recent years is an important adjunct to the production of new varieties (Hughes, 1949). At the present time varieties are not normally discarded because of their susceptibility to a disease, although this does happen on occasions, but rather, varieties are withheld from areas where particular diseases occur, and if the reaction of a variety is known, suitable precautions can be talcen when it is grown in the presence of a disease. In most cases varieties are not tested until they are approaching the commercial stage, although with leaf scald in North Queensland varieties are now being tested at an earlier stage in an endeavour to select resistant varieties for the leaf scald areas. The gradual elimination of many of the major diseases from commercial cane fields in Queensland has made it essential to carry out trials with such diseases in areas well isolated from cane growing areas, and fortunately, the Bureau of Sugar Experiment Stations has a suitable site at its Pathology Farm some eleven miles from Brisbane (Hughes, 1953). Trials which are carried out at the Pathology Farm can be divided into two main groups, namely : (I) Those in which the varieties under test are artificially inoculated with the disease causing agent, and (2) Those in which plots of diseased cane are planted in a regular pattern throughout the frial so that each variety is exposed to an even amount of natural infection. All trials are planted in the spring, i.e. about September. Varieties for inclusion in trials are propagated at the Pathology Farm in a special area set aside for the purpose. This isisolatedfrom the trial areaby about IOO m of trees and scrub which make an effective barrier against air-borne spores and insects. Consequently diseases usually do not spread into it. However, regular inspections are made and if odd diseased stools 'appear they are destroyed. The propagating of varieties for trials at the farm obviates the movement of large quantities of planting material, and varieties can be cut for trials as required. Ten setts of each variety are planted. These are despatched from each Experiment Station the year prior to their anticipated year for inclusion in trials..

2 1134 PATHOLOGY GROUP I. ARTIFICIALLY INOCULATED TRIALS Gumming Only varieties reaching the commercial stage are now tested for gumming resistance, since this disease has not occurred in Queensland cane field? for many years. Varieties under test are planted in 20 sett plots, and every third row is planted with alternate setts of the susceptible varieties H.Q.426 and M.189. Inoculation is carried out about February when hot humid conditions usually prevail. This is done by pricking a suspension of the bacterium into the young unfurling leaves with a bunch of rather coarse needles which lacerate them fairly extensively. Inspections are made in the growing crop at regular intervals and records of symptoms are kept. The following code has been used in recording results: I = No streaks; 2 = Few streaks; 3 = Many streaks; 4 C1-ilorosis; 5 = Wilting; 6 = Slight death; 7 = Much death. At maturity the stalks are cut and topped well below the growing point, then stacked in bundles and covered with tops and trash overnight to allow oozing to take place. Next day records are made of the numbers of stallcs oozing gum at either or both ends of the stalk, together with the total number of living stalks and stalks killed by gum. In assessing the susceptibility of a variety the results are compared with those of the standard varieties in the adjoining section of row. Leaf Scald Leaf scald trials are carried out both in Brisbane and at Myola near Cairns. The Brisbane trial is for the routine testing of varieties approaching commercial propagation, while at Myola an attempt is being made to select resistant varieties for the areas of North Queensland where leaf scald is endemic. Both trials are conducted on similar patterns; varieties to be tested are planted in two plots each of 20 setts and standard varieties of known susceptibility are distributed througl~outhe trial. Inoculation is carried out at the time of harvesting the plant crop which is about September. The cane is burned in the usual way before harvest and is cut two or three inches above ground with a sloping cut facing the south, i.e. away from the sun. Inoculum is prepared by mixing freshly extracted juice from diseased stalks with an equal volume of water. This is applied to the cut surfaces,of the stubble by means of a plastic wash bottle. All stalks in the trial are inoculated. No attempt is made to cover the freshly inoculated surfaces, and comparisons with cane which has been cut in the leaf, and in which the stubble has been covered with tops and trash after inoculation, have shown this to be unnecessary. Inspections are made at regular intervals throughout the growth of the crop and records kept of the numbers of stools infected and symptoms effhibited according to the following code: I. Few streaks, inconspicuous; 2. Few streaks, obvious; 3. Many streaks; 4. Scalding; 5. Side shootinp; 6. Jhlorosis; 7. Death;,,?. Acute stage. I In assessing the susceptibility of a variety consideration is given to the number of stools infected, the persistance of the disease and the severiiy of symptoms in comparison with the standard varieties.

3 D. R. L. STEINDL 1135 method (Bell, 1935) at planting time; however the amount of infection was frequently unsatisfactory, and after some experimental work the present method was adopted. Another change which has been made is the omission of non-inoculated plots, since under the conditions of our trials, very little natural spread takes place, and results from it are not significant. Red Rot Since red rot causes serious losses only in the Central and Southern districts of Queensland varieties from these areas are tested during their later stages of selection, whilst those from themorenortherly districts are tested only when they reach commercial status. Varieties to be tested are planted in two plots each of 20 setts with standards distributed throughout. Inoculum is prepared from freshly isolated cultures of the fungus which are sporulating abundantly, by breaking them up with water in a blender. It has been found that cultures kept in vitro for any length of time frequently lose their spore-producing ability and their virulence. Inoculation is carried out about September, when the cane is approximately twelve months old, and usually 20 stalks per plot are inoculated. The stalk is punctured with a sharp screwdriver at a distance of about one-third its length from thebase. The cut is made at right angles to the length of the stem so that the rind and fibres are severed. The inoculum is then introduced into this hole by means of a plastic wash bottle. No covering is put over the wound, as this has been found unnecessary, but if any rind is protruding from the stem it is pushed back into place. Several methods of puncturing the stalk and introducing the inoculum have been tried, but the above method has been found the simplest, and as reliable as any other. Examination of the trial is carried out about the end of December. A sloping cut is made through each internode of all inoculated stalks and a record is kept of the number of internodes which have become infected from the point of inoculation, together with the spread of the disease within the internodal tissue. In addition, any natural infection through growth cracks, nodal tissues or wounds, is recorded. Stallts which have been killed by the disease are recorded as such, but are not sliced. As with other disease trials the degree of susceptibility is determined by comparison with known standards. If a variety is as susceptible as Co.zgo it is not released for planting in Central or Southern Queensland. However, most of the very susceptible seedlings are eliminated through natural infection during the early stages of selection on the Experiment Stations. Mosazc A; in the case with red rot, mosaic is of importance only in the Central and Southern districts, -so varieties are tested for these two diseases at the same stage in their se-.lection. The varieties are planted in three plots each of ten setts, and are inoculated about mid-january when the primary shoots are cm high. If inoculation is carried out much earlier than this, varieties which show field resistance may become heavily infected, and on the other hand if i~loculation is delayed too long no infection will take place. The method used for inoculation is that described by Sein (1930) Although

4 I 1136 PATHOLOGY somewhat laborious it has the advantage that the inoculum does not age in the field as extracted juice does, since diseased leaves can be kept fresh in a plastic wrapping. Counts of diseased stalks and stools are made at intervals during the growth of the plant crop, and the degree of susceptibility is determined by comparison with known standards., ( Ratoon Stunting All varieties approachillg commercial propagation are tested for their reaction to ratoon stunting disease, and for the internal symptoms wl~ich are produced when they are infected. Inoculum consists of freshly exhressed juice from diseased stalks diluted with an equal part of water. Setts aredht squarely at tl~ends and immediately inoculated by the pressure cup method (Bell, 1935). The inoculated setts ire planted in plots of 15, and the same number of healthy setts are planted in the row beside them for comparison. Two such pairs of plots are planted with each variety. All inoculated plots are planted in the same rows and all healthy plots in adjoining rows to avoid the possil~ility of mecl~anical transmission from diseased to healthy cane along the rows. At harvest time knives are sterilized at the commencement of each row. The trial is examined in the mature plant and ratoon crops. Yield losses due to the disease are estimated to the nearest 10 per cent and both inoculated and healthy stallts are sliced and examined for internal nodal symptoms. Intensity of these symptoms is expressed in a scale of o to 3, in which Q.28, the standard, is ratedz. A syinptom rating below I could not be used for diagnosing the disease in the field, unless known healthy cane was available for comparison. GROUP 2. NATURALLY INFECTED TRIALS 1 Downy ~nildew Downy mildew is in the same category as gumming in that it hasn't been present in Queenslaild cane fields for a number of years. However, the disease is maintained at the Patl~ology Farm and all varieties reaching commercial status are tested for their reaction to it as a precaution against its re-occurrence. Because of the highly infectious nature of downy mildew the trials are carried out in an area of the farm which is separated from the main cane area by about I50 m of dense timber and undergrowth which acts as an effective barrier against the, movenzent of air-borne spores. Varieties are planted in three plots each of 20 setts. A z m plot of diseased eke is planted alternately with each plot and a I m plot at each end of the rows, as illustrated X Variety X X Variety X X Variety X X Variety X Variety Variety Variety Variety t y x a r e 1 : 1 variety X = I m of infected cane. :? 1 variety $ $ 1 i f 1 About the end of December or early January a row of mixed hybrid maize varieties is planted beside each row of cane. The maize plants become infected very quickly from the diseased cane, and act as an additional source of infection along the rows.

5 D. R. L. STEINDL 1137 Inspections are made during the growth of the crop and the numbers of diseased stalks and stools counted. This disease has been practically eliminated from commercial cane fields and the only recent occurrences have been ill the small southern mill areas. Trials include new commercial varieties and a few advanced seedlings which might be suitable for these areas. Varieties under test are planted in 3 plots, each of 15 setts in g m of row. Infection plots, each 4.5 m long aye alternated with the variety plots. The infection plots and variety plots are staggered in adjacent rows as illustrated in the plan below: -- Infection I Var~ety Infection I Variety -- Variety I Infection Var~ety I Infection 1 ~ar~ety Infection, I Variety Kassoer has been found the most satisfactory variety for infection plots, and is tl~e main variety used since it grows fairly well when infected and is attractive to the vectors. However, small amounts of other varieties are frequently included. Each infection plot is planted with about ten setts of diseased cane and two or three setts of healthy Kassoer which will provide infection material for the following year. Setts for the infection plots are talcen from well grown stallis in diseased stools. These may or may not be showing symptoms at the time of cutting but most will develop the disease, and will make a reasonable amount of growth. If stunted stalks are used they usually grow very poorly. Best results have been obtained by allowing the plant crop to grow for two years before harvesting. If cut at one year many of the infected stalks appear to be removed before the stubble is infected, and consequently the young ratoons remain healthy. Examinations are made in the ratoon crop and all diseased stools recorded. REFERENCES BELL, A. F. (1935) TWO inoculation methods. PYOC. Inter. Soc. Sug. Cane Tech., 5th Congr., : HUGHES, C. G. (1949) The production and testing of sugar cane seedlings. Bur. Sug. Exfit. Stat., Queensland, Techn. Comm. No. 2: HUGHES, C. G. (1953) The Bureau Pathology Farm. Cane Growers' Quart. Bull., Queensland, 17: SEIN, F. (1930) A new mechanical method for artificially transmitting sugar cane mosaic. J. De$t. Agr. Puevto Rico, 2: Discussion L. J. LIU (Puerto Rico) : How did you evaluate the degree of resistance to RSD? On the basis of yield or on the basis of yield and symptoms? B. T. EGAN (Australia): For ratoon stunting disease we found that there is no general relation between the degree of susceptibility and the type of symptoms. Our resistance rating is based almost entirely on yield. But we always make a note as to the type of symptoms. Field men make an'examination for RSD. This is most important, we find. Any more questions?