Growing Methods and Chemical Drenches Control Calla Soft Rot

Growing Methods and Chemical Drenches Control Calla Soft Rot P.J. Wright and G.E. Clark J. Koolaard New Zealand Institute for New Zealand Institute for Crop & Food Research Limited Crop & Food Research Limited Cronin Road, RD1 Private Bag 11 600 Pukekohe Palmerston North New Zealand New Zealand Keywords: Zantedeschia, Erwinia carotovora, chemical control, agronomic methods Abstract Bacterial soft rot, caused by Erwinia carotovora subsp. carotovora, is the most important disease affecting callas grown in New Zealand. The aim of this experiment was to determine the effects of chemical drench treatments on the incidence and severity of soft rot in callas grown in soil and in sawdust. There were four drench treatments: (1) no drench; (2) one drench application immediately after planting; (3) drench applied after planting, then at 28-day intervals; (4) drench after planting, then at 28-day intervals from the first visible occurrence of soft rot plot. The soil drench was an aqueous solution of 0.05% Kocide (350 g/kg copper), 0.05% Terrazole (350 g/kg etridiazole), and 0.15% Terrachlor (750 g/kg quintozene). Five litres of drench solution per plot was applied to the soil around the base of each plant. More calla plants died when grown in soil than in sawdust for each drench treatment. For both growing media, drenching plants at 28-day intervals resulted in the lowest numbers of dead plants during crop growth and rots in lifted tubers. INTRODUCTION Calla (Zantedeschia spp.) is a flower crop that is important to the New Zealand economy. Calla currently ranks second in revenue export earnings of flower crops after orchids, and was worth NZ$ 9.7 million in 2001 (Statistics NZ). Bacterial soft rot, caused by Erwinia carotovora subsp. carotovora (Jones, 1901; Bergey et al., 1923) is the most important disease affecting callas grown in New Zealand (Wright, 1998). All parts of calla plants can be affected by the disease, which can occur at any stage of plant growth (Wright, 1998). Erwinia carotovora subsp. carotovora survives in the soil on plant debris, and is often regarded as an opportunistic pathogen that infects plants primarily through wounds and natural openings such as lenticels when conditions that favour infection occur (Agrios, 1969; Walker, 1952). Erwinia carotovora subsp. carotovora often follows fungal pathogens (e.g. Rhizoctonia spp., Fusarium spp., Pythium spp., Phytophthora spp.) into plant tissues (Chupp and Sherf, 1960; Wright, 1995). Control measures for calla soft rot include discarding diseased tubers before planting, growing callas in well-drained growing medium at moderate temperatures (<24 C), careful management of irrigation, and use of chemicals as dips before planting or as drenches during crop growth (Kuehny et al., 1998; Wright, 1999). Drenching the soil around infected plants with fungicides has been used to control soil-borne fungal diseases of several flowering bulb crops (Moore, 1979). Beckman and Lukens (1997) recommended drenching calla tubers in pots with a broad spectrum fungicide 1-3 days after planting. Wright (2002) reported that drenching plants at 14-day intervals during crop growth with a solution containing copper hydroxide, quintozene and thiram reduced the incidence of calla soft rots. The growing method can also affect the incidence and severity of calla soft rot. Chen and Lin (2000) reported that calla plants grown in artificial media had lower levels of bacterial soft rot than those grown in soil. Wright (2002) also demonstrated that callas grown using the sawdust-shadecloth method had a lower incidence of soft rot during the growing season than callas grown in soil. The sawdust-shadecloth growing method involves placing tubers on nylon mesh shadecloth (which has first been laid on the soil), Proc. IX th Intl. Symp. on Flower Bulbs Eds.: H. Okubo, W.B. Miller and G.A. Chastagner Acta Hort. 673, ISHS 2005 769

then covering the tubers with a thick layer of sawdust. The calla roots grow through the shadecloth into the soil to obtain nutrients and water, and the tuber in the sawdust stays out of direct contact with the soil. This paper reports the effects of chemical drench treatments on the incidence and severity of soft rot in callas grown in soil and in sawdust-shadecloth beds. MATERIALS AND METHODS Calla tubers (30-40 mm diam.) of the cultivar Black Magic were planted on 11 October 2000 at Crop & Food Research, Pukekohe, New Zealand. The soil type was a Patumahoe mottled clay loam (ph of 6.3). Base fertiliser application of 15% potassic superphosphate (1 t/ha) was applied to the experimental site 6 weeks prior to planting to bring the levels of potassium and phosphorus in the soil within the range recommended for callas by Burge et al. (1994) (K: 1.4-2.1 me/100 g; P: 40-80 µg/ml). Plant spacing was 200 mm x 200 mm. Tubers planted in soil were hand-planted 100 mm deep using a trowel. For the sawdust-shadecloth method, 1.5 m x 1.5 m pieces of woven nylon wind break (38-45% porosity) were placed on the soil. The wind-break cloth was covered with approximately 20 mm of untreated pine sawdust onto which tubers were placed. A further 80 mm of sawdust was then placed on top of the tubers. Calcium ammonium nitrate (100 g/m 2 ) was spread over the sawdust to counteract any nitrogen deficiency that might occur as the sawdust decomposed. The sawdust-shadecloth plots were irrigated immediately after planting until the sawdust was fully wet, whereas the soil plots were not watered at this time. Irrigation was through drip tape (drippers 100 mm apart). The drip tape was placed on the beds between each row of plants. Irrigation began at early crop emergence on 10 November 2000 and was not applied after 12 March 2001. The evapotranspiration rate for the Pukekohe region in summer is c. 5 mm per day (30-35 mm over 7 days). For the soil only treatment, 30 mm of water was applied when less than 25 mm of rain had been recorded over a 7-day period. The sawdust-shadecloth method required a different irrigation regime because the higher, raised beds of free-draining sawdust dried out more rapidly than the heavy clay loam soil beds. For the sawdust-shadecloth method, 30 mm of water was applied at 7-day intervals regardless of rainfall. The experiment site consisted of eight beds, 10 m long. Treatment plots measured 1 m long x 1 m wide and were separated by unplanted buffer zones of 1 m. There were five plots in each row. The experiment site was divided into two blocks of four beds with each block containing 20 plots. In one block callas were grown in soil, in the other block callas were grown in sawdust (sawdust-shadecloth method). Two separate blocks were necessary because the two growing methods required different irrigation regimes. Each of the eight treatments had five plots (replications), which were randomised within each block. There were four drench treatments: 1. no chemical, 2. one drench application immediately after planting, 3. drench after planting, then again at 28-day intervals, 4. drench after planting, then again at 28-day intervals following the first visible occurrence of soft rot in the plot. The soil drench was an aqueous solution of 0.05% Kocide (350 g/kg copper), 0.05% Terrazole (350g/kg etridiazole), and 0.15% Terrachlor (750 g/kg quintozene). Drench solution (250 ml) was applied to the soil around the base of each plant (5 litres of drench solution per plot). Disease Assessments Plants were assessed for symptoms of soft rot on four occasions (11 December 2000, 8 January 2001, 5 February, and 5 March 2001) using a four-category scale: 0 = no soft rot symptoms, 1 = <50% of the plant had symptoms, 2 = >50% of the plant had symptoms, and 3 = plant completely dead. The first symptom of soft rot was usually a collapsed leaf (Wright, 1998). Sixteen soft-rotted petioles (two from each treatment) were 770

selected at random on each assessment date for isolation of soft rotting bacteria. Isolates that showed pectolytic activity on crystal violet pectate medium (Cuppels and Kelman, 1974) were identified using the methods outlined by Lelliott and Stead (1987). On 13 March 2000, after the foliage of all plants had completely died and shrivelled, the tubers were lifted, weighed, and assessed for the incidence of soft rot using the following scale: 0 = no soft rot, 1 = <10% of tuber rotted, 2 = 10-25% of tuber rotted, 3 = 26-50% of tuber rotted, 4 = >50% of tuber rotted. Sixteen soft-rotted tubers were selected at random at each harvest date for isolation of causal organisms. The percentage of symptomless plants, as well as the percentage of dead tubers, were calculated and analysed using analysis of variance to compare treatments. RESULTS Erwinia carotovora subsp. carotovora was isolated from the healthy-diseased margin of all petioles sampled on the four assessment dates, and from all tubers sampled after lifting. The incidence of calla soft rot increased steadily during crop growth, with the rate of increase of the number of soft rotted plants varying for each growing medium and drench treatment (Fig. 1). In all eight treatments, levels of symptomless plants (disease category 0) were around 90% on 11 December 2000. On 5 March 2001, symptomless plants ranged from 8% to 10% for plants grown in soil or sawdust and not drenched, to 67% for plants grown in sawdust and drenched after planting and at 28-day intervals. Plants drenched at 28-day intervals (treatment 3) had more (P>0.05) symptomless plants from January until March for the soil medium, and from February to March for the sawdust medium. The level of symptomless plants for sawdust-grown plants remained higher for longer in the season than those in soil, especially for treatment 2. For both growing methods, drenching plants at 28-day intervals during crop growth (treatment 3) resulted in the lowest levels of plant death (disease category 3) from February and March, with levels lower (P<0.05) than for both non-drenched plants (treatment 1) and for plants that were drenched only after planting (treatment 2) (Fig. 2). Plants that were drenched at 28-day intervals during the growing season from when disease was first found in the treatment plots (treatment 4) had more (P<0.05) dead plants in February and March than treatment 3 in soil, but not sawdust. The effect of drench treatments on the incidence and severity of tuber rot in lifted tubers differed for plants grown in the two media (Table 1). The number of tubers with no symptoms of rot (rot score 0) in March was lowest when no drench was applied (treatment 1), with 8% symptomless tubers lifted from the soil and 12% from sawdust-shadecloth. Tubers from plants drenched after planting and at 28-day intervals during the growing season (treatment 3) had the lowest levels of bad (rot category 3) for both soil (15%) and sawdust-shadecloth (13%) growing media. The overall disease score for treatment 3 was lower (P<0.05) than for all other treatments. The treatment with the next lowest overall disease score was treatment 4, when plants were drenched at 28-day intervals during the growing season following the first visible occurrence of soft rot in the plots. DISCUSSION Several strategies combined in an integrated manner are likely to give optimum control of soil-borne diseases (Powelson et al., 1993). Two of the most important strategies, exclusion and irradication of inoculum, were investigated in our experiment. The experiment clearly demonstrated that calla soft rot can be reduced using the sawdust-shadecloth growing method, which helped exclude the soil-borne bacterial pathogen from the tubers, and by regular drenching plants with bactericide-fungicide solutions, which helped to eliminate inoculum present around the roots, tubers, and bases of the growing plants. 771

ACKNOWLEDGMENTS Financial support for this research was provided by the New Zealand Foundation for Research, Science and Technology. We acknowledge the assistance of Mo Jeram for field assistance. Literature Cited Agrios, G.N. 1969. Plant Pathology. Academic Press, New York. Beckman, P. and Lukens, T. 1997. Simple steps for pot calla success. GrowerTalks 60:49, 54. Burge, G., Dennis, D., Spiers, M. and Clark, C. 1994. Crop nutrition. In: New Zealand Calla Council Grower s Handbook, New Zealand Calla Council Inc. Chen, C.W. and Lin, C.Y. 2000. Control of Erwinia soft rot disease of calla lily. Plant Pathol. Bull. 9:107-114. Chupp, C. and Sherf, A.F. 1960. Vegetable Diseases and Their Control. The Ronald Press Company, New York. Cuppels, D. and Kelman, A. 1974. Evaluation of selective media for isolation of soft-rot bacteria from soil and plant tissue. Phytopathology 64:468-475. Kuehny, J.S., Holcomb, G.E., Chang, W. and Branch, P.C. 1998. Chemical treatments to control Erwinia soft rot of calla rhizomes. HortTechnology 8:353-356. Lelliot, R.A. and Stead, D.E. 1987. Methods in Plant Pathology. Volume 2. Methods for the diagnosis of bacterial diseases of plants. British Society for the diagnosis of bacterial diseases of plants, British Society for Plant Pathology, Oxford, Blackwell Scientific Publications. Moore, W.C. 1979. Diseases of Bulbs. Reference Book HPD 1. Ministry of Agriculture, Fisheries and Food, Her Majesty s Stationery Office, London. Powelson, M.L., Johnson, K.B. and Rowe, R.C. 1993. Management of Diseases Caused by Soilborne Pathogens. In: R.C. Rowe (ed.), Potato Health Management, APA press, St Paul, Minnestoa, United States. Walker, J.C. 1952. Diseases of onion, garlic, leek, chive, and shallot. p.225-262. In: Diseases of Vegetable Crops, McGraw-Hill Book Company, New York. Wright, P.J. 1995. Controlling soft rot in callas. Commercial Horticulture 43:64. Wright, P.J. 1998. A soft rot of calla (Zantedeschia spp.) caused by Erwinia carotovora subspecies carotovora. New Zealand J. Crop Hort. Sci. 26:331-334. Wright, P.J. 1999. Microorganisms cause rot. Horticulture News March 1999:17. Wright, P.J. 2002. Three ways to control calla soft rot. NZ Calla News, Edition 41, February 2002. 772

Tables Table 1. Proportions (%) of calla tubers in different soft rot severity categories for tubers harvested (13 March 2000) from different growing method and soil drench treatments. 0 = no soft rot, 1 = <25% of tuber rotted, 2 = 26-50% of tuber rotted, 3 = >50% of tuber rotted. Growing method Drench treatment Disease category Overall disease score 0 1 2 3 Soil 1 8 19 26 47 2.12 2 18 25 19 38 1.77 3 44 26 15 15 1.01 4 35 19 13 33 1.44 Sawdust 1 12 29 29 30 1.77 2 30 19 26 25 1.46 3 67 11 9 13 0.68 4 42 23 16 19 1.12 LSD (P=0.05) for comparing overall disease scores: 0.23 (df=32). Figures 100 80 Symptomless plants (%) 60 40 20 Sawdust Trt.1 Sawdust Trt.2 Sawdust Trt.3 Sawdust Trt.4 Soil Trt.1 Soil Trt.2 Soil Trt.3 Soil Trt.4 0 11 Dec 8 Jan 5 Feb 5 Mar Date Fig. 1. Mean proportions of calla plants without soft rot symptoms during crop growth for plants grown using two different methods (sawdust or soil), and treated with four different soil drench treatments. Bars for each date are LSDs (P=0.05). 773

60 50 Dead plants (%) 40 30 20 Sawdust Trt.1 Sawdust Trt.2 Sawdust Trt.3 Sawdust Trt.4 Soil Trt.1 Soil Trt.2 Soil Trt.3 Soil Trt.4 10 0 11 Dec 8 Jan 5 Feb 5 Mar Date Fig. 2. Mean proportions of dead calla plants during crop growth for plants grown using two different methods (sawdust or soil), and treated with four different soil drench treatments. Bars for each date are LSDs (P=0.05). 774