Clubroot control in Pukekohe

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1 New Zealand Journal of Agricultural Research SSN: (Print) (Online) Journal homepage: Clubroot control in Pukekohe A.G. Watson To cite this article: A.G. Watson (1965) Clubroot control in Pukekohe, New Zealand Journal of Agricultural Research, 8:4, , DO: / To link to this article: Published online: 16 Jan Submit your article to this journal Article views: 102 View related articles Citing articles: 2 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 24 December 2017, At: 20:55

2 988 CLUB ROOT CONTROL N PUKEKOHE By A. G. WATSON Plant Diseases Division, Department of Scientific and ndustrial Research, Auckland (Received 22 luly 1965) ABSTRACT Field experiments were carried out to determine a suitable method for controlling club root of cabbages in Pukekohe. Control methods were evaluated by their efficacy both in reducing clubbing and in increasing yields. The best control was achieved by dipping the roots in a calomel slurry so that 0.05% g calomel was applied per plant. PCNB (0.1 g per plant) and captan (0.1 g per plant) applied in the transplant water both gave good control. The effect of soil-moisture on loss of yield caused by the disease was evaluated and a control measure which involves increasing the soil moisture during the growing season to a constant high level is suggested. NTRODUCTON Club root disease caused by the fungus Plasmodiophora brassicae Wor. is a major disease of cabbage and cauliflower crops in the Pukekohe district near Auckland. n an earlier paper (Watson 1965), reviewed briefly the published control methods and concluded that soil application of fungi-toxic materials was likely to be the most effective single control method for the disease in Pukekohe. Seedbeds are maintained free from the disease by using new ground or by periodic fumigation, but infection commonly occurs in the field after transplanting. deally, whole fields should be treated with a fungi-toxic chemical to elimina~ the fungus. However, this method is impracticable. Other investigators have achieved practicable control by applying fungicides during transplanting to the small volumes of soil in which the young roots grow. With existing cultural practices in the area, there are two convenient ways of applying the fungicide; either dissolved or suspended in water applied to each plant immediately after transplanting, or as a slurry in which the transplant roots are dipped. The individual grower will be influenced in his choice of application method by the degree of clubroot control achieved and by the particular facilities available to him. N.Z. agric. Res. 8: 988~96

3 A. G. WATSON 989 A large number of fungi-toxic materials and concentrations have been suggested for clubroot control. The three most successfully used materials, applied to the volume of soil occupied by the young plant root system, are mercuric chloride (W(\}lker, Stakmann, and Pryor 1944; Colhoun 1960; Brandenburg 1962; Colhoun 1963), mercurous chloride (Walker et az. 1944; Colhoun 1960; Colhoun 1963), and pentachloro nitrobenzene (Gallegly and Bishop 1955; Rosser 1957; Campbell 1957; Brandenburg 1962; Catovic-Catani and Rich 1964). The concentrations recommended by these investigators range from 0.06 g to 0.28 g mercuric chloride per plant, 0.05 g to 0.15 g mercurous chloride per plant, and 0.2 g to 1.7 g pentachloronitrobenzene (PCNB) per plant. Even allowing for variations in fungicidal effect on P. brassicae caused by variations in such factors as spore load in the soil, soil temperature, and soil ph (Colhoun 1953), the range of concentrations recommended by different workers is very large. n this paper report the results of field experiments designed to determine the most efficient method of controlling clubroot disease in Pukekohe. MATERALS AND METHODS Two experiments were carried out at Pukekohe near Auckland on a field where 100% clubroot infection of a cabbage crop was observed in the season prior to the first experiment. The soil had the following characteristics: ph 6., field capacity 41 %, cation exchange capacity 27.4 (m.e. %), base saturation 53.5%, loss on ignition 17.2%, and C : N ratio 13.7%. The cabbage variety "Golden Acre", known to be highly susceptible to the disease (Catovic-Catani and Rich 1964), was used in both experiments. The materials used in the treatments were: 98% mercurous chloride (calomel), 20% mercurous chloride W.P., mercuric chloride (commercial), hydrochloric acid (commercial), bentonite, perlite (fine grade), methyl cehulose, "Lanstan" (l-chloro-2-nitropropane), captan (N-trichloromethylmercapto-4-cyclohexene 1, 2-dicarboximide) as "Orthocide" 50% W.P.), and PCNB (pentachloronitrobenzene) as "Terrachlor" 75% W.P.). The formulations prepared from these materials and used in the experiment S are shown in the tables of results. The materials were applied either in the transplant water which was poured into the soiil around the plant roots immediately after transplanting, or as a slurry in which the transplant roots were dipped. Use of the slurry method involved immersing the roots of bundles of about 30 plants in the slurry and agitating so that each root system was well coated. Both experiments were laid out in such a way that replicate treated rows were separated by untreated control rows. Apart from the experimental treatments, the grower followed normal cultural procedure in both experiments. The crops were examined several times for any differences in appearance caused by the different treatments. When each crop was harvested (over a period

4 990 Clubroot control of eight weeks), the cut stems and roots were left in position in the field. After the last cut had been taken, the severity of clubbing on the roots and the number of uncut heads were recorded for each replicate treatment. have expressed the severity of clubbing by an indexing method similar to that used by Colhoun (1963). Each root was pulled from the ground, and the amount of clubbing was graded according to the following scale:- _ 2 (0) No visible clubbing. (1) One or two small clubs on the tips of the lateral roots. (1) Several small clubs on the lateral roots. (2) Large clubs on the lateral roots. (3) Slight clubbing of the tap root, many large clubs on the laterals, and slight reduction of the root system. (4) Considerable clubbing of the tap root and much reduced root system. (5) Complete clubbing of the whole tap root and virtually no lateral roots. 4 2 Fig. 1.-The grades of clubbing used for obtaining the disease indices.

5 A. G. WATSON 991 This range of infection is illustrated in Fig. 1. The disease indices of the different replicates were obtained by multiplying the percentage of plants in each grade by the grade number and adding the products. EXPERMENT AL AND RESULTS Experiment 1 was planted 25 February 1963, and the results were recorded on 28 June. There were four replicates of 70 plants in each treatment. All the treatments slightly retarded plant growth for the first three weeks. Experiment 2 was planted 28 February 1964, and the results were recorded on 20 July. There were three replicates of 450 plants in each treatment. Treatments incorporating PCNB and "Lan stan" retarded growth slightly at first, and from the seventh week onwards the untreated control rows became noticeably yellow and stunted. The results of both experiments, together with treatment formulations and application methods, are shown in Table 1. Daily rainfall totals during the periods of the two experiments were taken, and from these, and from estimated daily losses of water from the soil, soil-moisture deficit curves were constructed (Fig. 2). The significance of soil moisture is noted in the Discussion. DSCUSSON The application rates for the materials used in experiment 1 were taken from recommendations by other investigators. They all retarded plant growth slightly but gave good control of club root. Tn experiment 2, application rates were adjusted following the results of my laboratory experiments (Watson 1965). From these experiments concluded that infections by P. brassicae do not occur, during the first month after treatment, under field conditions in soil which has been treated with 100 p.p.m. calomel, 200 p.p.m. captan, or 200 p.p.m. PCNB. n order to prevent infections during the first month after transplanting, the soil which the cabbage roots occupy at one month must be treated with the fungicides at the above rates. The average monthold root system was found to occupy an approximate cylinder of soil weighing about 500 g at field capacity. n experiment 2, therefore, the fungicides were applied with the object of treating this mass of soil at the desired rates.'-' Neither application method distributes the fungicide evenly through the whole 500 g of soil; most of the fungicide is applied immediately around the root system by both methods. Early infections of the tap roots have a more damaging effect on the plant than later infections of the lateral roots (Colhoun 1963). Hence with both application methods the fungicide concentration gradients which are established with respect to both space and time ensure better control of the more damaging type of infection. The slurry used in experiment 1 was found to be unsatisfactory, for two reasons: it failed to leave a consistently heavy deposit on the

6 Treatments Calomel slurry (20 g calomel, 130 g Bentonite, 1 L water) Calomel in transplapt-water (10 g calomel W.P. in 15 L 150 c.c. per plant) Acid mercuric chloride in transplant-water (10 :; HgC12 30 g HC in 15 L; 150 c.c. per plant) PCNB in transplant-water (50 g PCNB in 15 L 150 c.c. per plant) No treatment TABLE 1. Disease indices for Plasmodiophora brassicae and percentage uncut cabbage heads following fungicide applications during transplanting for two seasons, 1963 and Estimated Estimated active Disease index % uncut active material and S.B. and S.B. Treatments per plant material per plant 0.1 g Calomel slurry (50 g 109 ± ± 1.8 calomel, 840 g perlite, 25 g methyl cellulose 0.05 g 9,085 c.c. water) PCNB/captan slurry 0.1 g 188 ± ± (100 g PCNB g captan 840 g perlite, 25 g methyl cellulose 9,085 c.c. water) Captan in transplant 0.1 g 166 ± ± 0.9 water (10 g captan in 15 L, 150 c.c. per plant) 0.1 g of each material 0.1 g PCNB in transplant water 0.5 g 175 ± ± 0.7 (10 g PCNB in 15 L, 150 c.c. per plant) 0.1 g "Lanstan" in transplant water (113 g "Lan stan" in 0.14 g 45,430 C.c. 284 c.c./plant) ± ± 1.3 No treatment Disease index % uncut and S.E. and S.E. 83 ± ± ± ± ± ± 3.0! 152 ± 3 14 ± ± 8 19 ± 1.0 T 401 ± ± 7.6 \0 \0 tv Q :::: ~... \:) \:)... <":> \:) ;::: ~ i

7 A. G. WATSON 993 dipped roots; and the heavy particles of calomel rapidly sedimented. The methyl cellulose + perlite slurry used in experiment 2 was better in both respects, though agitation of the mixture was periodically necessary to maintain the calomel in suspension. The amount of active material applied to each plant depends upon the composition of the slurry and the amount adhering to each plant. Though the amount deposited will vary slightly with different transplants, the standard formulation should give field control in all cases. n 1963, untreated cabbages developed severe root clubbing, but the loss of yield was only 9.3% of the possible maximum. n 1964 clubbing was again severe, but the loss of yield was 51 %. The factors which influence loss of yield through disease were examined to determine the reason for this difference. The disease effect on crop yield is dependent upon the inoculum potential (sensu Garrett 1956) and also on the expression of infection by the plants. The inoculum potential of P. brassicae towards a host plant is determined by the following factors:- (i) The susceptibility of the host. (ii) The number of infective zoospores coming into contact with the host roots. (iii) The biotic and physical conditions prevailing in the soil. These factors determine the amount of infection which takes place, and they have been described in detail by Colhoun (1953, 1958). n addition, the factors which determine the expression of infection by plant yield are:- (i) The tolerance of a particular host to the infection. (ii) The time in a plant's life and position on its root system that infection take place. (iii) The biotic and physical conditions in the soil following infection. n the two experiments carried out in different years the only factor which differed appreciably was the soil moisture/time curve. Fig. 2 shows the soil water deficit through the growing season calculated empirically from the rainfall figures. The loss from the soil occupied by the cabbage root system on rainless days has been given the value 0.1 in. This value is based on Gabites' (1956) application of Thornthwaite's ndex (Thornthwaite 1948) of water loss from a vegetative cover of turf. Two distinct patterns are evident. n 1963, planting was followed by 29 days of moderate water deficit and then by three months with only short periods of low water deficit. n 1964, planting was followed by 19 days with almost no water deficit and then a period of 48 days of increasing water deficit. Soil moisture is known to have the following effect on P. brassicae infection and its expression by plant yield. High soil moisture favours heavy infection (Colhoun 1953), but it also enables even heavily infected

8 994 Clubroot control DAYS AFTER TRANSPLANTNG Fig. 2.-Soil moisture deficit below field capacity during the 1963 and 1964 experiments. plants to grow almost normally. Low soil moisture does not favour infection, but it causes even slightly infected plants to suffer from lack of water. The soil moisture level varies through the season, and this complicates the situation further because:- (i) Early infections have a more severe effect on the yield than late infections. Early infections often cause clubbing of the tap root because then the epidermal cells and root hairs in this region are susceptible, being young and full of protoplasm (Samuel and Garrett 1945). Later infections are more likely to be situated at the expanding susceptible periphery of the root system. (ii) The time in a plant's life during which it is subjected to water stress is a factor which determines the effect of that water stress on yield. Thus if the soil moisture level is fairly low in the period immediately after transplanting and then increases to a constantly high level, the loss of yield through disease will be small. This was the situation in But if the soil moisture level is high immediately after planting and then decreases for a period while the plant is growing, the loss of yield will be great. This was the situation in A control measure utilising this interaction between soil moisture and time on infection and expression of the disease, could be applied in areas of the world where predictable rainfall trends or irrigation make it possible to plant into a fairly dry soil in which the soil moisture will gradually rise to a constant high level. Further work on the minimum soil moisture level required for infection and the duration of the period when early infection should be prevented wi make the requisites for this control method more exact. Catovic-Catani and Rich (1964) have suggested ridging up around the stems so that adventitious roots grow out through soil where the moisture level is not favourable for infection.

9 A. G. WATSON 995 CONCLUSONS Good control of club root can be achieved in the Pukekohe district by applying fungicides during transplanting. Calomel, applied by dipping the transplant roots in a slurry, gives the best control. This method can be used by growers who transplant by hand or by machine. The cost of treating 1 acre using 0.05 g calomel per plant is between 7 and 8. This sum is small compared with both the total cost of producing 1 acre of cabbages (about 150) and the price realised by the grower for a good crop (about 200). The disease can reduce yield by at least 50% in a bad clubroot year. Acidic mercuric chloride is effective but its corrosive and extremely toxic nature does not recommend it for use. f a grower prefers to apply the fungicide in the transplant water, thus avoiding the dipping process and the additional labour it requires, then PCNB at 0.1 g per plant gives good protection. Captan used at 0.1 g per plant gives slightly inferior control compared with PCNB. Field results were in close agreement with the results of my laboratory experiments (Watson 1965) using the root-hair count method (Samuel and Garrett 1945). The laboratory method can be used, therefore, to determine the effect of soil fungicides against clubroot infection in the field. ACKNOWLEDGMENTS wish to thank Mr D. McDougal of Pukekohe for his co-operation in allowing us to use his land for the experiments, and Messrs B. T. Hawthorne and H. Gillard and Mrs A. Watson for help in the field. REFERENCES BRANDENBURG, W. 1962: Chemical control of brassica clubroot. N.Z. comml Grow. 18 (6): CAMPBELL, L. 1957: Control of clubroot of cauliflower. Phytopathology 47: 518 (abstract). CATOVC-CATAN, S.; RCH, A. E. 1964: The effects of three chemicals and cultural practices on control of Plasmodiophora brassicae. P. Dis. Reptr 48: COLHOUN,. 1953: A study of the epidemiology of clubroot disease of brassicae. Ann. appl. Bioi. 40: : Clubroot disease of crucifers caused by Plasmodiophora brassicae. Phytopath. Pap pp : Club root disease of crucifers-some recent developments. Gdnrs' Chron. 147: ~ : Control of club root in brassicas. P. Path. 12: GABTES,. P. 1956: The estimation of natural evaporation and water weed. Proceedings of the conference on soil moisture held at the Dominion Physical Laboratory. N.Z. Dep. scient. indo Res. nt. Ser. 12: GALLEGLY, M. E.; BSHOP, C. R. 1955: Pentachloronitrobenzene for control of clubroot of brassicas. Pl. Dis. Reptr 39:

10 996 Clubroot control GARRETT, s. D. 1956: "Biology of the root infecting fungi." pp Cambridge University Press. 293 pp. ROSSER, W. R. 1957: Control of clubroot of Brassicae. Plant Path. 6: SAMUEL, G.; GARRETT, S. D. 1945: The infected root hair count for estimating the activity of Plasmodiophora brassicae in the soil. Ann. appl. Bioi. 32: THORNTHWATE, C. W. 1948: An approach towards a rational classification of climate. Geogri Rev. 38: WALKER, J. c.; STAKMANN, M. A.; PRYOR, D. E. 1944: Efficacy of fungicidal transplanting liquids for control of clubroot of cabbage. Phytopathology 34: WATSON, A. G. 1965: Toxicity and persistence of certain fungicides against Plasmodiophora brassicae Wor. N.Z. Jl agric. Res. 8: