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Vol. 4. No.4, pp. 463-469, 1996 The effect of different tillage systems on the yield and incidence of root and stalk rot of sorghum B.C. FLETT Grain Crops Institute, Agricultural Research Council, Private Bag X1251, Potchefstroom 2520, Republic of South Africa (Received 27 March, 1996; accepted 24 September, 1996)
Code Number: CS96087
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ABSTRACT The effect of different tillage systems on grain yield, root and stalk rot of sorghum (Sorghum bicolor (Moensch) L) was studied over four seasons at Potchefstroom. Seasonal effects were apparent for all parameters, indicating significant environmental effects. Tillage treatments had no significant effects on grain yield and root and stalk rot of sorghum. Positive correlations existed between grain yield and root rot at the soft dough stage and root rot and stalk rot at soft dough and harvest. Key Words: Grain yield, root rot, Sorghum bicolor, stalk rot, tillage RESUME
L'effet de plusieurs systemes appliques sur le rendement de grain, racine et tige du sorgho (Sorghum bicolor) etait etudie pendant quatre saisons a Potchfstroom. Les effets saisonniers etaient presque apparents au niveau de tous les Parametres, indiquant les effets considerables de l'environement. Les traitements pendant la culture n'ont pas eu des effets considerables sur les rendements des grains, racines et tiges du sorgho. Les correlations positives ont existe entre le rendement des grains, racines et tige pendant le stade doux du ble et la recolte. Mots Cles: Le rendement des grains, la pourriture des racines, Sorhum bicolor, la pourriture des tiges, la culture INTRODUCTION Root and stalk rots of sorghum (Sorghum bicolor (Moensch) L.) cause crop losses worldwide (Mughogho, 1984). These diseases reduce root development, water and nutrient uptake, and increase lodging (Pappelis and BeMiller, 1984). Sorghum in South Africa is produced under dry land conditions in areas with highly fluctuating rainfall regimes (Anon., 1991 ). Sorghum in these production areas is thus often subjected to heat and drought related stresses. Root and stalk rots are stress related diseases which increase with suboptimal growing conditions (Ho and Melhus, 1940;Chambers, 1982). DouPnik(l984)reported that practices which reduce plant stress reduced root and stalk rots of sorghum, these include minimum tillage.
The high risk associated with sorghum production in South Africa, due to fluctuating climatic patterns, has resulted in the increased use of minimum tillage. Minimum tillage can have a positive effect on soil moisture conservation, reduction of input costs (Mannering and Fenster, 1983) and reduced wind and water erosion (Doupnik, 1984). Minimum tillage systems involve the retention of crop stubble on the soil surface (Blevins, et al., 1983). Crop stubble influences the incidence and severity of diseases by supplying a nutrient source for survival and production of disease inoculum (Cook, et al., 1987). Minimum tillage has, in many cases, increased diseases where crops have been grown in monoculture (Doupnik, 1984).
Conflicting results on the effect of tillage on root and stalk rot and grain yield of maize have been obtained (Duley, 1957; Parker and Burrows, 1959; Hargrove, 1985; Lipps and Deep, 1991). The effect of tillage practices on sorghum production have previously been extrapolated from maize results (Doupnik, 1984). A minimum tillage rotation system developed for sorghum production in the semi-arid Central Great Plains in the United States has reduced the incidence and severity of stalk rot while increasing yield (Doupnik, 1984). The success of this ecofallow system was ascribed to increased soil moisture and lower, more constant soil temperatures. Thus the effect of tillage systems on the grain yield of monoculture sorghum and the concomitant possibility of increasing root and stalk rot was studied. This is the first published report on the effect of tillage systems on these factors in sorghum production. MATERIALS AND METHODS The trial was conducted on a clay soil (ca. 30 %) at Potchefstroom for four consecutive seasons from 1988/89 to 1991/92. Prior to the first experimental season (i.e. 1987/88) the land was ploughed, disced and the seedbed prepared. Three tillage systems were replicated four times and laid out in a randomized block design in 30 x 40 m plots. The conventionally tilled treatment was prepared with an early summer mouldboard plough and disced prior to planting. Two minimum tilled treatments were prepared: the first with a chisel plough and then disced, the second with an early summer chisel plough which was planted directly. The experiment was planted to the sorghum cultivar NK283. Fertilizer [2:3:2(22)] was applied at 18.85 kg N, 28 kg P and 18.85 kg K ha^-1 before planting. Weeds were controlled with atrazine sprayed 2 weeks after crop emergence. An insecticide mixture of demeton S-methyl EC and endosulfan EC was used to control aphids and stalk borers.
Percentage root and stalk discolouration was visually determined on 20 randomly sampled plants (main culms) per treatment at the soft dough stage and two weeks prior to harvest. Ears of the three central rows (10 m) were hand harvested, threshed and weighed to determine grain yield. Climatic variables were measured over all seasons for the duration of the study. Soil fertility variables (NH4^+, No3^-, P, K, pH and C) were measured at initiation and completion of the study.
Data were analyzed using a factorial analysis of variance (Statgraphics 2.6) with season (whole plot) and tillage (subplot) as factors. All data were subjected to log transformation. Correlations were used to determine the relationships between the variables measured. RESULTS No significant tillage x season interactions were obtained. Significant seasonal effects for root and stalk rot incidence at both sampling times and grain yield are provided in Table 1. Rainfall, and maximum and minimum temperatures for each season are presented in Tables 2 and 3 respectively. Root rot at the soft dough stage was significantly higher in the 1989/90 season than the 1988/89 season. The inverse was apparent where root rot at harvest in the 1988/89 season was significantly higher than 1991/92 and 1989/90 seasons. Stalk rot at the soft dough stage was significantly lower in the 1991/92 season than previous seasons. Stalk rot at harvest was significantly higher in 1988/89 than the following seasons.
Soil C, pH and K did not differ significantly over tillage treatments and time (data not shown). NH4^+, No3^-, P, K, pH and C levels differed significantly over time with increases from .67 and 4.47 ug g^-1 to 5.58 and 13.53 ug g^-1 respectively. A significant phosphate x tillage treatment interaction was obtained (Table 4).
Root and stalk rot incidence at both sampling times and grain yield did not differ significantly under the different tillage systems (data not shown). TABLE 1 - Incidence of root rot, stalk rot and grain yield of monoculture sorghum over various seasons, averaged over treatments
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Season Soft dough Harvest Grain
--------------------- ---------------------- ----------
Root rot Stalk rot Root rot Stalk rot Yield
% discol- % discol- % discol- % discol- t ha^-1
ouration ouration ouration ouration
---------------------------------------------------------------------------
1988/89 30.0a - 66.8c 67.1b 1.390a
1989/90 45.7c 33.7b 44.7a 49.0a 2.045ab
1990/91 42.9bc 41.3b 60.8bc - 2.793b
1991/92 33.9ab 16.8a 46.8ab 49.7a 1.944ab
---------------------------------------------------------------------------
P <.0001 <.0001 <.0001 <.001 <.0001
CV% 16.9 16.9 12.7 12.3 4.3
---------------------------------------------------------------------------
Means followed by different letters in columns differ significantly at
P=0,05 according to Scheffes multiple range tests.--------------------------------------------------------------------------- TABLE 2. Monthly rainfall (mm) figures for seasonal duration of the study
-------------------------------------------------
Month Season
--------------------------------------
1988/89 1989/90 1990/91 1991/92
-------------------------------------------------
October 122.9 16.1 10.7 116.8
November 62.6 146.7 13.4 15.9
December 72.1 77.1 59.6 74.4
January 134.9 47.3 135.3 24.3
February 157.0 162.8 92.8 8.6
March 33.8 62.0 62.8 17.8
April 65.2 103.8 3.0 35.7
Total 636.5 615.8 377.6 293.5
-------------------------------------------------
TABLE 3. Monthly minimum and maximum temperature (C) figures for seasonal duration of the study.
------------------------------------------------------------------------
Month Season
-----------------------------------------------------------
1988/89 1989/90 1990/91 1991/02
----------- ----------- ------------ -------------
Min Max Min Max Min Max Min Max
------------------------------------------------------------------------
October 13.3 28.7 12.6 27.2 13.3 28.7 12.6 27.2
November 15.2 31.2 13.6 29.2 15.2 31.2 13.6 29.2
December 16.3 30.7 14.8 28.7 16.3 30.7 14.8 28.7
January 17.4 28.2 15.6 32.8 17.4 28.2 15.6 32.8
February 15.6 28.1 15.6 33.0 15.6 28.1 15.6 33.0
March 14.3 25.8 12.7 29.7 14.3 25.8 12.7 29.7
April 8.3 25.5 10.6 27.0 8.3 25.5 10.6 27.0
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Positive correlations existed between root rot and stalk rot at the soft dough stage (P=0.69) and harvest (P=0.87), and grain yield and root rot at the soft dough stage (P=0.72) (Table 5). DISCUSSION The significant seasonal differences obtained for grain yield, root and stalk rots in this study can be ascribed to the seasonal and within season fluctuating environmental and soil fertility factors measured. Environmental factors that affect host susceptibility and the pathogen complex of root and stalk rot include soil moisture (Chambers, 1982; Rao et al., 1987; Pande et al. 1990), soil temperature (Sumner and Dowler, 1983), soil nutrient status (Ho and Melhus, 1940; Chambers, 1982) and soil physical properties (Sumner and Bell, 1986). Insect and nematode damage (Palmer and Kommedahl, 1969; Warren and Kommedahl, 1973; Norton, 1985) and viral or fungal leaf infections (Tu and Ford, 1971; Fajemisin and Hooker, 1974) also affect the disease complex.
Phosphate levels were reduced in the plough plots over time. However, no effect on root, stalk rot or yield could be related to this. These reduced levels of phosphate could be due to burial of stubble during ploughing. Mackay et al. (1987) found phosphorous levels to increase in the upper soil layers under reduced tillage maize treatments. Should sorghum, in this study, have utilized similar levels of phosphorous over all tillage treatments, this would manifest itself as an observed drop in phosphorous in ploughed plots.
TABLE 4. Season x tillage interaction table for phosphate levels (mg kg^-1 ) measured at the beginning and end of the study ------------------------------------- Tillage treatment 1987 1991 ------------------------------------- Plough/disc 32.15 16.33 b Chisel/disc 29.68 27.51 a Chisel 27.15 26.52 a LSD n.s - CV% 6.01 -Means followed by different letters in columns differ significantly at P=O.05 according to Scheffes multiple range tests ------------------------------------------------------------------------- TABLE 5. Correlation coefficients of variables measured in a monoculture sorghum tillage trial
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Parameter Soft dough Harvest
--------------- --------------
Root Stalk Root Stalk
rot rot rot rot
-------------------------------------------------------------
Grain yield 0.72* 0.46 -0.06 -0.58
(12) (9) (12) (9)
Soft dough Root rot 0.69* -0.38 -0.45
(9) (12) (9)
Stalk rot 0.47 0.04
(9) (6)
Harvest Root rot 0.87*
(9)
* Correlations significant at P<0.05
-------------------------------------------------------------
Non-significant root and stalk rot incidences between the tillage systems can possibly be attributed to differences in above mentioned variables under and within the different tillage systems. The results obtained in this study are not consistent with previous studies on maize where root and stalk rot was reduced under minimum tillage (Parker and Burrows, 1959). Fusarium spp., the major fungal genus isolated from sorghum roots and stalks on previous studies on the same soil type (Moolman, 1992), increased in maize subcrown mesocotyls and crowns under minimum tillage (Lipps and Deep. 1991). The increased surface stubble and potential disease inoculum in minimum tillage systems (Cook et al., 1987) may be outweighed by factors which reduce predisposition to root and stalk rot, such as moisture stress reduction (Mannering and Fenster, 1983). Reduced stalk rot observed in the ecofallow system described by Doupnik (1984) may be due to the crop rotation aspect rather than minimum tillage. The reduction of stress due to the implementation of reduced tillage systems to control sorghum root and stalk rots (Doupnik, 1984) are not supported by this study.
The positive correlations between root and stalk rot at the soft dough stage and harvest supports previous reports that root rots precede stalk rots (Whitney and Mortimore, 1957; Williams and Schmitthenner, 1963; Sumner, 1986). The positive correlations between grain yield and root rot at the soft dough stage may be explained by the ability of sorghum to compensate for damage through tillering (Van Rensburg and Van Den Berg, 1992).
Previous reports on the effect of tillage on maize grain yield have been inconsistent (Duley, 1957; Hargrove, 1985) but implementation of minimum tillage systems have been shown to reduce production costs and are more economically viable when compared to conventional tillage (Mannering and Fenster, 1983). No sorghum yield differences were obtained in this study.
Maize produced under minimum tillage systems have been reported to increase locally important diseases such as Diplodia stalk rot (Byrnes and Carroll, 1986), Diplodia ear rot (Stenocarpella maydis .(Berk.) Sutton) (Flett and Wehner, 1991 ), Gibberella ear rot (Fusarium graminearum Schwabe) (Windels and Kommedahl, 1984) and Grey leaf spot (Cercospora zeae-maydis Tehon and E.Y. Daniels) (De Nazareno et al., 1992). Minimum tillage has been reported to increase sorghum seedling diseases under low soil pH and temperature conditions (McLaren, 1987). The other important local sorghum disease, ergot, has not been reported to be increased by minimum tillage. The use of minimum tillage for sorghum production under adequate soil nutrition and optimum planting dates will therefore not increase local sorghum diseases. Therefore, greater economic benefits will be obtained due to reduced production costs when minimum tillage is used in sorghum production.
Climatic and other environmental inconsistencies complicate disease prediction, so preventative control measures need to be taken to reduce root and stalk rots. Tillage studies have to be repeated over a number of localities with different soil, climatic and inoculum conditions to confirm the results obtained in this study. Further research is required to determine the role of pathogens in disease succession, the role of pathogen toxins and root damage on yield loss, and the interacting environmental factors affecting root and stalk rot. ACKNOWLEDGEMENTS The technical help of P.J. Minnie, G. Mokgatsi, J.G.C. Kroukamp and K. van der Merwe is gratefully acknowledged. REFERENCES Anonymous. 1991. Report for the financial year ended 31 December 1990. Grain Sorghum Board, Pretoria. Blevins, R.L., Smith, M.S., Thomas, G.W. and Frye, W.W. 1983. Influence of conservation tillage on soil properties. Journal of Soil and Water Conservation 38:361-385. Byrnes, K.J. and Carroll, R.B. 1986. 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