M. Mezzalama, K.D. Sayre, and J. Nicol

In the Yaqui Valley (Sonora, northwest Mexico) the most common crop sequence is wheat planted as a winter irrigated crop on conventionally-tilled raised beds, followed by maize as a summer crop. Tillage is often accompanied by burning of crop residues, although some maize and wheat straw is baled-off for fodder and some is incorporated during tillage.

Farmers are intensely interested in new production technologies that would markedly reduce tillage operations and retain crop residues, because they could reduce their production costs and lead to more sustainable soil management.

For all their benefits, these practices may introduce some problems. Reduced tillage and residue retention can foster build-up of soil-borne pathogens that cause root rot and plant parasitic nematodes. Poor root health is a major factor contributing to inefficient use of water and nitrogen fertilizer by wheat grown under reduced tillage.

In 1999 we started monitoring root rot on wheat in two long-term experiments initiated in 1992. The objective of these experiment was to investigate the production feasibility of growing wheat using farmers' practices versus wheat sown on beds initially formed for the first crop and then reused with only superficial reshaping (permanent beds) before planting each succeeding crop.

The aim of the monitoring is to assess the effect of adopting reduced-till bed planting in irrigated systems on wheat root rot pathogens.

Materials and Methods

The trials are sited at CIANO research station of Ciudad Obregon, Sonora, Mexico, and were initiated in 1992.

Experiment 402 was designed as a randomized complete block with four replications and a split-plot arrangement. Main plots consisted of two tillage treatments: permanent beds (PB) and conventionally tilled beds (CTB); two subplots of straw management: retention (PB-straw; CTB-straw) and burning (PB-straw burned; CTB-straw burned). The experiment involves a two-crop annual rotation with wheat planted in November and harvested in early May and maize planted in early June and harvested in October. Plot size is 8 m long x 8-10 beds wide (each bed 75 cm).

Experiment 209 was designed as a randomized complete block with three replications and a split plot treatment arrangement. Main plots consisted of five tillage straw treatments; seven subplots of N fertilizer applications of urea. The experiment involves a two crop annual rotation with wheat planted in November and harvested in early May and either soybean or maize planted in late May or early June respectively and harvested in October. Plot size is 13 m long x 8 beds (75 cm each).

Experiment 209. Wheat on permanent beds, all residues retained		Experiment 209. Wheat on permanent beds, all residues removed
Experiment 209. Wheat on permanent beds, all residues burned

Root rot evaluation was carried out on three out five main plots and three subplots as follows:

During 1999 and 2000, three months after planting, 15 plants/plot were sampled for fungal root rot lesion evaluation. Seminal, crown, and tiller roots were scored on a scale of 0 to 7 (Thomashow and Weller, 1988). Plant parasitic nematode Pratylenchus thornei was extracted from soil (using one 200-g composite homogenous soil sample with the whitehead tray method) and from wheat roots.

Results

The mean yield of the experiment was significantly greater in 1999 than in 2000. In 1999 the yield obtained on PB-straw was significantly greater than burning straw, while in CTB-straw there was no significant difference. In 2000 yield was significantly greater under CTB than PB (Table 1, Figure 1).

Table 1. Yield in experiment 402. Treatment Grain yield at 12% H2O Mean yield in 1999 7806.56 Mean yield in 2000 6318.50 significantly different at P<05 Mean yield PB-straw in 1999 8174.75 Mean yield PB-burn in 1999 7464.5 significantly different at P<0.05 Mean yield PB in 2000 5873.25 Mean yield CTB in 2000 6763.75 significantly different at P<0.5

In this experiment root rot incidence was significantly greater in 1999 than in 2000, although always at a very low level (2.71 on 0-7 scale). In 2000 the incidence of root rot was greater under PB than CTB and in CTB-straw burned than CTB-straw. No other significant difference was found (Table 2, Figure 2).

Table 2. Root rot incidence on wheat crown roots in experiment 402.  Treatment Score (0-7 scale)  Mean score in 1999 2.71  Mean score in 2000 1.88 significantly different at P<0.05  Mean score PB in 2000 2.45  Mean score CTB in 2000 1.31 significantly different at P<0.05  Mean score CTB-straw in 2000 0.75 Mean score CTB-burn in 2000 1.87 significantly different at P<0.05

Yield in 1999 was significantly greater than in 2000. In 1999 PB-straw burned yield was lower than in CTB- straw and PB-straw chopped. Also in 2000 yield was lower in PB-straw burned than in PB-straw chopped (Table 3, Figure 3).

Table 3. Yield in experiment 209. Treatment Grain yield at 12% H2O Mean yield in 1999 6183.14 Mean yield in 2000 5000.25 significantly different at P<0.05 Mean yield PB-straw burned in 1999 5625.77 Mean yield CTB-straw in 1999 6399.88 significantly different at P<0.05  Mean yield PB-straw burned in 1999 5625.77 Mean yield PB-straw chopped in 1999   6523.77 significantly different at P<0.05 Mean yield PB-straw burned in 2000 47667.88 Mean yield PB-straw chopped in 2000 5286.77 significantly different at P<0.05

Root rot incidence on seminals was higher in 2000 than 1999. In 1998 the summer crop was soybean, while in 1999 it was maize. This may explain the higher incidence of root rot on wheat in 2000, as maize can carry pathogens (i.e. Fusarium spp.) that may affect wheat roots. In 1999 the incidence was significantly higher in PB-straw burned than in CTB-straw incorporated. In 2000 the incidence was significantly higher in PB-straw chopped than in CTB-straw (Table 4, Figure 4).

Table 4. Experiment 209. Root rot evaluation on seminals. Treatment Score (0-7 scale) Mean score in 1999 1.19 Mean score in 2000 2.85 significantly different at P<0.05 Mean score PB-straw burned in 1999 1.70 Mean score CTB-straw in 1999 0.89 significantly different at P<0.05 Mean score CTB-straw in 2000 2.30 Mean score CTB-burn in 2000 3.31 significantly different at P<0.05

Number of Pratylenchus thornei extracted from wheat roots
The population of P. thornei extracted from roots was significantly greater in 1999 than in 2000. In 1999 the population was significantly higher in PB_straw burned than in CTB-straw. No significant difference was found in year 2000 (Table 5, Figure 5).

Table 5. Experiment 209. Number of Pratylenchus thornei extracted from wheat roots. Treatment No. of P. thornei/plant Mean number in 1999 776.88 Mean number in 2000 49.62 significantly different at P<0.05  Mean number PB-straw burned in 1999 1099.77 Mean number CTB-straw in 1999 430.22 significantly different at P<0.05

Number of Pratylenchus thornei extracted from soil (0-20 cm)
The mean number of nematodes 200g-1 of soil_dw was not significantly different in the two years of monitoring. In 1999 no significant difference was found among treatments, while in 2000 the population was higher in PB-straw burned than in CTB-straw and in PB-straw chopped (Table 6, Figure 6).

Table 6. Experiment 209. Number of Pratylenchus thornei extracted from soil (0-20 cm) Treatment No. of P.thornei 200g-1 soil(dw) Mean number PB-straw burned in 2000 60.11 Mean number in 2000 6.88 significantly different at P<0.05 Mean number PB-straw burned in 2000 60.11 Mean number CTB-straw chopped in 2000 27.7 significantly different at P<0.05

Conclusions

Under the environmental conditions of northwest Mexico it is possible to conclude that:

• Straw retention is a critical practice in the adoption of reduced tillage and of conventional tillage:

• On yield to ensure long term production sustainability, increasing soil organic matter and improving soil physical conditions.

• On fungal and nematode root rot agents increasing the beneficial soil microflora and enhancing natural biocontrol.

• Straw burning did not show a significant effect on the control of fungal root rot pathogens and nematodes; therefore it does not seem justified, as it is in cooler and more humid areas, where moisture in soil can favor fungal pathogens (i.e. Gaeumannomyces graminis tritici, Rhizoctonia solani, Pythium spp.) not present in this area (Cook, 1992).
• The low incidence of root rots in both experiments did not explain a yield reduction in 2000, considering also that in experiment 402 root rot incidence was higher in 1999 than in 2000 CTB.
• Although the evaluation of plant root pathogens shows some significant effects with management treatments, none of these are directly related to yield.

© CIMMYT July 2001

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