By G. ROTH Introduction
A new disease of sugarcane was reported in the Midlands region of Natal in April, 1964. The symp- toms became most apparent and damage greatest during the following winter months, after which they became less pronounced. The infection was named 'Wartburg disease' after the area in which it was first found and the varieties affected were N:Co.376, N:Co.293 and Co.331.
Infected plants exhibited poor growth, development was stunted and in some cases individual stools were killed. Mortality was greatest in the variety N:Co.376, followed by N:Co.293. Following early reports of this infection, studies of the disease were undertaken by staff at the Experiment Station, Mount Edgecombe.
Symptoms
The cane most seriously infected by the disease was 6 to 7 months old plant or ratoon crops. Infection originates in several distinct and quite independent centres in the field and spreads along the cane line causing large gaps to appear as stools die off (Plate
1, A).
The disease attacks the plant in an early stage of growth, infecting the young tillers at ground level (Plate 1, B). Infected plants remain small in size and are characterized by the presence of abundant dry leaves and leaf sheaths. The infected stool displays symptoms of starvation and later it may collapse com- pletely. During the initial stage of infection, leaf sheaths on the lower part of the stalk turn reddish in colour on the outside, but when they are stripped off, a white fungus mycelium is exposed. Where infection has penetrated at a node, water soak marks are to be found on the rind extending downwards from the node and leaf scar (Plate 2, A). Microscopic exami- nation has shown that the fungus exists in greatest concentration in the peripheral vascular bundles at the internodes, and in the sclerenchyma lying below the epidermal tissue of the stalk.
In some circumstances, symptoms of the disease may differ from those described and brown discolor- ation of the nodes and internodes may occur. When this happens, the leaf symptoms and the effect of the disease on the growth of the cane, differ very little from those already described. This variation in colour is thought to be due to differences in the populations of associated micro-organisms and the presence of another fungus, which will be discussed later.
Preliminary Field Surveys
In May, 1964, a survey was conducted in the Natal Midlands in an attempt to determine the intensity of infection and the susceptibility of varieties to 'Wart- burg disease'. Of 93 fields on 33 farms in the Wart- burg area, 7 fields, planted with the varieties N:Co.376 and N:Co.293, were severely infected. Inspection of
the Eston-Powers Court area revealed that of 58 fields on 20 farms, 9 fields planted with N:Co.376, 5 fields with N :Co.293 and one field with Co.331 were infected with the disease.
It appears from the information available that the variety N:Co.376 is particularly susceptible to this disease, while N:Co.293 is next in its susceptibility.
Unfortunately, these varieties are widely grown in the Midlands and their future has been questioned in the event of the disease becoming serious. The prospect of being able to recommend alternative varieties must remain in abeyance until more has been learned about the disease itself and data obtained concerning its life history and development.
The Search for the Causative Organisms Infected cane plants, obtained from several fields, were lifted complete with their root systems and taken to the laboratory. Plate cultures were made following superficial sterilisation of the infected and diseased parts of the cane. The various fungi which developed were isolated in pure cultures and then identified. The following were isolated: Colletotrichum fakatum Went.;
Melanconium iliau Lyon.; Fusarium moniliforme Sheld.;
Epicoccum nigrum Link; Thielaviopsis paradoxa (De Seynes) v Hohnel; and one non-sporulating fungus producing a cottonwool-like mycelium.
In infected cane, in which the stalks were dis- coloured dark brown, perithecia were found similar to those of the genus Gnomonia. By separating this fungus, using the general agar plate technique, Melan- conium was isolated in several instances.
The Pathogenicity of Isolated Fungi
Fungi isolated from diseased cane were used to inoculate several hundred two- and three-bud setts.
These artificially infected setts were kept in glass jars under conditions of high humidity. In another experi- ment, single bud setts were planted in sterilised soil in earthenware pots, the buds being inoculated with the various fungi cultured on an agar medium. In both these preliminary trials it was shown that only the non-sporulating fungus induced a pathogenic reaction, damaging the bud and surrounding tissue in the same way as the disease in the field. In view of this, it was decided that more detailed studies should be carried out on the nature of the pathogenicity of the disease producing organism.
Field Experiments
A common means of determining the disease resis- tance of a variety is to plant it in an area where plants of the same crop have recently succumbed to the disease. It is assumed that if the fungus causing the disease lives in the soil, the area, particularly if it is covered with trash, is highly infected. This practice has been followed in the Wartburg area on Windy Hill Estates. However, in spite of the heavy soil
Proceedings of The South African Sugar Technologists' Association—March 1965 183
PLATE 1
(A) An early stage of field infection by 'Wartburg disease' or 'stem rot'.
(B) Two healthy, mature stalks of cane and several younger stalks which are damaged by the fungus.
Note the early symptoms of infection on the young shoots at ground level.
184 Proceedings of The South African Sugar Technologists' Association—March 1965
infection, no symptoms of the disease appeared during the present season (Plate 2, B). Despite this, it would be unwise to suppose that these results will be dupli- cated under other cultural conditions, as there may be different strains of the fungus, each with different pathogenic properties.
At Mount Edgecombe a field experiment was put down in which seed cane, infected with 'Wartburg disease', was used. Four varieties were involved, namely N:Co.376, Co.331, N:Co.293 and N.53/216, and in each case the buds on each sett were artificially inoculated with the mycelium of 'Wartburg disease', grown in pure culture. In no case has the resultant plant shown any evidence of infection by the disease.
In view of the results obtained from these various trials it is considered that the disease is induced not by the infective fungal organism alone, but by its association with a complex of climatic and soil environment con- ditions.
Glasshouse Experiments
On the 14th April, 1964, an experiment was laid down to determine the degree of pathogenicity of the stem rot fungus and the relative susceptibility of 4 different varieties of sugarcane. The trial, which was of randomised block design, comprised 4 replications of all combinations of 4 varieties and 3 superimposed treatments. Each treatment plot contains 10 single bud setts of seed cane, which means that 480 of these setts were used in the whole experiment.
The three disease treatments were: (a) untreated control, (b) the single bud on each sett infected arti- ficially with the 'Wartburg disease' fungus and (c) the bud on each sett similarly infected with the fungus and, after further incubation, dipped for 10 minutes in a 0.5% solution of Aretan. The buds in treatments (b) and (c) were infected by adding to each an equal quantity of a two-week-old fungus mycelium plus agar.
In all three treatments the seed cane was incubated in perforated plastic bags which were kept for 3 days in the dark in a laboratory.
The setts were planted in boxes and kept in warm humid conditions in glasshouse cubicles — the humi- dity varying from 85 to 100% and temperature ranging from 18° C. to 32° C. All the setts were watered every second day. The cane which developed
TABLE 1 TABLE 1
Mean percentage germination of single bud setts, following artificial inoculation with mycelium of the 'Wartburg disease'
fungus.
VARIETY
N:Co.293 . Co.331 . . N:Co.376 N.53/216 Means
CONTROL
(a) 90 0 95.0 92.5 97.5 93.8
FUNGUS
(b) 47.5 72 5 52.5 40.0 53.1
FUNGUS AND ' A R E T A N '
(c) 70.0 75.0 65 0 65.0 68.7
MEAN OF
(b) AND (C)
58.8 73.8 58.8 52.5
was grown in these conditions for 6 months and the records obtained are set out in Table 1.
It can be seen from the foregoing data that dipping the infected setts in 'Aretan', following a 3 day incubation period (treatment (c)) provides better ger- mination than when these setts are not so treated.
However, in both treatments, germination of setts is very much poorer than in the untreated disease-free control, indicating that inoculation with the disease has had an adverse effect. Germination of inoculated setts of the variety Co.331 is very much higher than is the case with other varieties in both treatments (b) and (c), suggesting possible resistance.
TABLE 2
Mean percentage mortality of plants six months after emergence.
VARIETY
N:Co.293 . . . Co.331 . . . . N:Co.376 . . . N.53/216 . . .
CONTROL
(a) 0 0 2.5 0
FUNGUS
(b) 36.2
7.8 5.0 58 2
F U N G U S 'AND A R E T A N '
(c) 7 . 0 3.5 7 . 2 0 . 0
A number of plants which developed from the ger- minated setts, died during the 6 months following emergence. These deaths, expressed as percentages, are quoted in Table 2. It is obvious from this table that mortality in the infected, but otherwise untreated plants (treatment (b)) is much greater in the case of the varieties N:Co.293 and N:53/216 than for N:Co.
376 and Co.331. Mortality among infected plants treated subsequently with 'Aretan' (treatment (c)), is much lower and differences between this and the control, or between varieties, did not achieve statistical significance.
The leaf sheaths of the plants that eventually died, first turned reddish in colour at the lower nodes. A ring cf white fungus mycelium then developed and this can be seen in Plate 1, B. Once these symptoms are visible, the plants die within a few days. Dissection of the plants in the control plots showed that the fungus was absent, whereas in the infected plants internal signs of fungus damage were obvious, the tissue being penetrated by fungal hyphae. The fungus was readily isolated from tissue of the diseased plants and when this was grown in pure culture it proved to be identical with the material used to infect the buds.
Another experiment, which was carried out under glass during the summer, involves 13 different varieties of sugarcane. The experiment was put down in November, 1964, using artificially inoculated cane setts, which were grown at a high level of relative humidity (70 to 100%). Despite this, no infection occurred during the first 3 months of plant growth, due perhaps to the high temperatures used. These were particularly high during the period of germina- tion, reaching 45° C. Certainly, the activity of the
Proceedings of The South African Sugar Technologists' Association—Marcli 1965 185
P L A T E 2
(A) Typical symptoms of reddish water-soak marks at various stages in their development.
Note the one healthy stalk on the extreme left.
(B) Young cane plants in an experiment laid down on heavily infected soil on Windy Hill Estate. No symptoms of the disease occurred.
TABLE 4
YIELDS: Varieties at differing ages in tons cane per acre
Tl 12 M O N T H CUTTING
N:Co.376 N:Co.382 N :Co.293 N:Co.310
P (12) 54.76 41.38 47.11 45.18
Rl (12) 38.03 33.78 38.58 31.02
R2 (12) 55.35 42.53 51.70 43.56
R3 (12) 37.65 33.64 33.93 31.24
Total Tl 185.79 151.15 171.32 151.00
T2 16 MONTH CUTTING
P (16) 57.24 54.74 53.18 51.07
R l (15) 62.60 58.58 60.62 54.51
R2 (17) 41.86 33.54 31 86 33.58
Total T2 161.65 146.86 145.66 139.16
T3 20 MONTH CUTTING
P (21) 69.44 60.09 56.63 57.12
R l (19) 60.64 57.25 56.80 50.66
R2 (S) 15 28 12.91 15.21 8.98
Total T3 145.36 130.25 128.64 116.76
T4 24 M O N T H CUTTING
P (24) 69.12 56.94 57.27 57.08
R l (24) 73.48 57.72 62.59 58 89
Total T4 142.60 114.66 119.86 115.97
TABLE 5
YIELDS: Varieties at different ages in tons sucrose per acre
Tl 12 MONTH CUTTING
N:Co.376 N:Co.382 N:Co.293 N:Co.310
P (12) 7.79 5.32 6.81 7.28
R l (12) 5.85 4.97 5.96 5.50
R2 (12) 8.46 5.83 7.32 7.14
R3 (12) 6.29 4.87 5.44 5.79
Total Tl 28.39 20.99 25.53 25.71
T2 16 MONTH CUTTING
P (16) 8.06 7.32 7.25 7.77
R l (15) 8.96 7.89 8.15 8.28
R2 (17) 7.17 5.25 4.97 5.66
Total T2 24.19 20.46 20.37 21.71
T3 20 MONTH CUTTING
P (21) 9.60 8.69 7.58 8.80
R l (19) 10.24
9.16 9.33 9.14
R2 (8) 2.10 1.56 2.01 1.44
Total T3 21.94 19.41 18.92 19.38
T4 24 MONTH CUTTING
P (24) 10.30
8.66 8.70 9.84
R l (24) 11 56
8.36 9.55 10.26
Total T4 21 86 17.02 18.25 20.10
190 Proceedings of The Smith African Sugar Technologists* Association- -March 1965
Proceedings of The South African Sugar Technologists' Association—March 1965 191
192 Proceedings of The South African Sugar Technologists'' Association—March 1965 TABLE 7
TIME OF CUTTING IN MONTHS
Plant Cane Tl 14 . . . . T2 16 . . . . T3 18 . . . .
1st Ratoon Tl 12 . . . . T2 14 . . . . T3 16 . . . . 2nd Uatoon T l 12 . . . . T2 14 . . . .
TONS CANE PER ACRE
68.2 67.5 76.6
54.2 62.6 81.4
55.3 57.4
TONS SUGAR PER ACRE
10.31 9.91 10.68
7.60 8.89 11.94
8.12 7.85
TONS C A N E PER A C R E PER M O N T H
4.87 4.22 4.25
4.52 4.47 5.09
4.61 4.10
T O N S SUCROSE PER A C R E PER M O N T H
0 . 7 4 0 . 6 2 0 . 5 9
0 . 6 3 0 . 6 3 0 . 7 5
0 . 6 8 0 . 5 6
TOTAL WATER
(Rain and irrigation)
IN INCH/ACRE
56.23 64.29 74.83
43.74 55.39 61.52
52.37 63.15
T O N S CANE PER INCH
WATER
1.2 1.1 1.0
1.2 1 .1 1.3
1.1 0 . 9
previous plant cane crop had been cut in February, 1962, and it may well be that this timing and growth period in relation to season, may have combined to affect yields in a manner as yet unknown.
Fibre % Cane
The percentage of fibre in cane is yet another factor influenced by age of the crop at harvest. The mean percentages of fibre in cane for various varieties har- vested at different ages, are shown in Table 8.
TABLE 8 Fibre % Cane
MEANS FOR TIMES
Tl . . T2 T3 . . T4 . .
N:Co.376 10.68 11.34 12.15 12.25
N:Co.382 13.99 14.65 15.88 16.30
N:Co.293 11.34 11.33 12.80 12.91
N:Co.310 10.58 11.44 12.38 12.59
MEAN
11.66 12.19 13.30 13.54
These figures indicate that the older the crop is when it is cut, the greater the fibre content of the cane, regardless of variety. However, when comparing cane cut at 12 and 24 months, the increase in fibre content for the varieties N:Co.376, N:Co.382, N:Co.293 and N:Co.310 is respectively 1.57%, 2.31%, 1.57% and 2.01 %. Despite this variation, it is clear that from the factory viewpoint there is an advantage in milling young cane.
Conclusions
{a) The older the crop the lower the yield of cane and sugar per acre, per unit of time.
(b) Cane should be harvested earlier than at present whenever the resultant increase in yield more than compensates for the additional cost of handling two crops.
(c) To make best use of the maximum growth period each year, harvesting at 12 month intervals is most desirable. This harvesting can be carried out at time of peak sucrose which, conveniently, is also the best time for development of ratoons.
(d) Lodging of cane should not be tolerated as n o t only does it cost more.to harvest and transport, but yields of both cane and sucrose are reduced.
(e) The younger the cane the easier it is to mill, as fibre % cane increases sharply with age.
Recommendations
Growers should strive to grow the bulk of their cane as a 12 month crop. In practice this means t h a t the largest possible area that can be cut on any farm, between August and November, should be harvested when the cane is about 12 months old. Of the remain- der, cane due to be ploughed out is best harvested in June and July. The balance, consisting of cane which, is more than 12 months old, but still as young as possible, should be cut to meet mill requirements earlier or later than the August-November season.
Proceedings of The South African Sugar Technologists'' Association—March 1965 193
Mr. Hempson: It is only possible to cut at twelve months if the crop is cut at the best time of the year.
Harvesting in December, January and February, even where growing conditions are good, is out of the question for twelve month cane.
At other times of the year we have found that cutting at twelve to fourteen months gives the best profit per acre per annum.
Some years ago, cutting at twenty-four months, we got up to 139 tons of cane per acre. In this last season we cut at between thirteen and fourteen months and averaged 52 tons per acre but with a much better sucrose content.
Mr. Pearson: I made certain recommendations at the end of my paper which are in accordance with your remarks.
Mr. du Toit: This question of harvesting frequency has been argued for many years.
I remember some time back that we were cutting Co.290 every year to 10th ratoon and getting about 30 tons to the acre.
If cane is cut annually at about September/October it is likely that its sucrose content will be as high, if not higher, than cane of eighteen months or two years being cut in September/October. But if the cane is cut in say May or February the sucrose content will be very poor.
Mr. Pearson's observation about trash is quite correct. Millers may not thank planters for growing a short crop as it might make them short of fuel.
If we accept that the weight of crop increases per unit of time as the age of the harvest decreases, we must not go beyond the optimum period.
If we decide to cut annually it will be. essential to make better use of moisture.
Mr. Thompson (in the chair): Shorter cropping should also give greater benefit from trashing.
194 Proceedings of The South African Sugar Technologists" Association—March 1965 Mr. Grice: The matter of economics again comes
into this. The amount of extra weeding required before a canopy forms must be considered and also the extra cost of labour for more harvestings.
Mr. Pearson: We did not go into costs because this was carried out in small replicated plots.
Weeds should only be a problem with plant cane as trashing should take care of the weeds later.
Mr. Hill: Tongaat carried out a cost investigation on annual cutting of cane, taking into account such factors as double acreage, weeding, increased fertilizer, easier cutting of young cane, bundling, etc. and found that it would pay handsomely when compared with cutting at two years.
Cutting N:52/ll at twelve months Tongaat got a better yield increase than for any other variety. In fact all our figures for twelve month cane were better than Mr. Pearson's.
We have had as much as 50% increase of two twelve months cuts over a twenty-four month cut.
However, our experiments were in small blocks which in many cases were isolated in a field of cane that had been harvested in a period of hot dry winds, so that the older cane deteriorated rather badly and had dried out.
Mr. Wilson: We have recently conducted an indus- trial survey at the request of the S.A. Sugar Associa- tion. One farmer about 40 miles from here, with a quota of 181 acres, on Dwyka soil has a sucrose quota of 821 tons. He cuts all of his cane at twelve to four- teen months, under dryland conditions.
Mr. du Toit: Under irrigation it is possible to cut a crop at twelve or fourteen months but it is not easily done under dryland conditions. Yet according to Mr. Pearson the percentage gain, cutting annually over a two year period, is greater for dryland than for irrigated cane, which seems strange.
Mr. Pearson: Table 7 gives times for cutting under irrigation, and it shows that growth is fairly similar in both the first and second year whereas under dry- land conditions there is far less growth in the second year. This seems to be the wrong way round. If you cut from August to October, as you would with a twelve month crop, for two months there is very little canopy and very little transpiration, but usually rain is falling and therefore the soil profile is built up with water. The crop can therefore take off from a full profile right through its growing period. A two year crop has been transpiring all through the second year of growth and the water status is only the water falling on the soil — there is no accumulation for the two or three months of no transpiration. The twelve month crop has more water to produce cane than the second year of the twenty four month crop.
Mr. Hempson: Some varieties do not come into Mr. Pearson's category at all. We have found N:Co.
382 on our porous soil responds extraordinarily well.
As Mr. Pearson pointed out, twenty or twenty-four month crops often contain dead cane, with a conse- quent loss in weight, money and growing time. Had the crop been cut younger there would be a consider- able gain. In N :Co.382 grown on porous soil, the percentage weight per stick of twelve month cane is considerably better than for older cane. It is also easier to cut and therefore requires a smaller labour force. Sucrose per ton of cane is better because the older cane tends to lodge, with a consequent drop in sucrose.
Mr. Pearson: We showed in figures produced a few years ago a drop of 2% in sucrose in lodged cane.
Mr. Johnson: In India we found a direct ratio between cane deterioration in the interval between harvesting and milling and the age of the crop, i.e.
older cane deteriorated much more rapidly. If there was any delay between harvesting and milling, due maybe to transport delays, cane more than eighteen months old was barely worth milling.
Mr. Gosnell: 1 have checked the results of the growth experiments and see they very largely confirm Mr. Pearson's current results. For the first eighteen months there was good uniform growth in the dryland section of the experiment, but from eighteen to twenty four months the increment in growth was very small.
The irrigated section did not show such clear trends, possibly because of inadequate replication, but it did appear that from about sixteen months, when lodging occurred, there was no effective further growth where- as in the period twelve to sixteen months the increment was as good as the dryland section.
Dr. Cleasby: Harvesting cane at a younger age will almost certainly become the practice in the foreseeable future. The advantage regarding yield has been realised for quite a long time. But the industry will be faced with major yield fluctuations. If we harvest at a b o u t eighteen months we will be cutting about two thirds of our land every year. In times of drought we could go from a very large crop to a very small crop. At 100% twelve month cutting, taking into consideration that rainfall can vary from 25 inches to 50 inches, the crop could vary by 100% from one year to another.
Mr. Thompson: As has been pointed out, all c a n e cannot be cut at twelve months as there must be flexibility of harvesting age. There is no doubt, h o w - ever, that if the average at present is twenty-five months it should be considerably less.