- Top of page
- Materials and methods
Establishment methods for rice crops in tropical Asia are very diverse, leading to variation in the structure of rice canopies. Differences in canopy structure can in turn affect the spread of the rice sheath blight pathogen, Rhizoctonia solani. Rice sheath blight epidemics were compared during two seasons in crops established by different methods: direct broadcasting of pregerminated rice seeds, and transplanting of rice seedlings at spacings of 20 × 20 cm, 13 × 25 cm and 25 × 25 cm between hills (i.e. along and between rows, respectively). In both years, the apparent infection rate based on incidence data and the terminal severity of sheath blight were lower in the direct-seeded crops than in any of the transplanted ones, regardless of spacing. The frequency of leaf-to-leaf contacts (CF) between hills (or plants) was highest in direct-seeded rice, and lowest in rice transplanted at a spacing of 25 × 25 cm. Larger CF is known to favour rice sheath blight epidemics. The apparent contradiction between higher incidence and lower CF in the transplanted stands than in the direct-seeded stands is interpreted in terms of accessibility of healthy host tissues to the spread of the pathogen in the canopy, and accounts for within-host (rice hill or plant) and between-host (hill or plant) disease spread. The analysis of incidence-severity relationships indicated a less aggregated distribution of the disease in direct-seeded rice, which was related to the spatial distribution of the tillers. These findings have direct implications for the management of the disease.
- Top of page
- Materials and methods
Sheath blight, caused by Rhizoctonia solani, has become an important disease of rice in tropical Asia, especially in intensive rice cropping systems ( Teng et al., 1990 ; Savary et al., 1994 ). Cultivars highly resistant to this pathogen ( Xie et al., 1992 ; McClung et al., 1997 ) are not currently available in Asia, and so far control of the disease has relied mainly on the use of fungicides, when affordable by farmers ( Savary & Mew, 1996). However, this option is not considered sustainable because of residue problems and the potential risk of emergence of pathogen populations that are resistant to fungicides. Other options to control sheath blight are necessary, and these include cropping practices that may minimize the disease.
Rice crop establishment methods in Asia are very diverse, and include direct seeding by broadcasting, and transplanting of seedlings at different spacings. These different crop establishment methods lead to differences in the structure of the rice crop canopy, i.e. in crop geometry. In the case of a direct-seeded crop established by broadcasting rice seeds, the crop consists of randomly distributed plants with three to five tillers each. Such a crop can thus be seen as a population of tillers that are slightly aggregated at a scale corresponding to the area occupied by one plant. In the case of transplanted rice, the crop consists of uniformly distributed hills, each having 10–40 tillers. A transplanted rice crop, by contrast, can thus be seen as a population of tillers that are strongly aggregated at a scale corresponding to the area occupied by one hill (which is usually larger than the area occupied by one direct-seeded plant).
During the leaf-borne phase of sheath blight epidemics, the fungus spreads by means of runner hyphae that originate from lesions and progress towards healthy tissues ( Ou, 1987). These healthy tissues may belong to the same tiller, the same ‘host aggregation unit’ (whether a transplanted hill or a direct-seeded plant), or a different unit. Contacts between infected and healthy tissues are therefore a prerequisite for disease spread. The frequency of leaf-to-leaf contacts represents an important epidemiological factor for sheath blight ( Savary et al., 1995 ; Castilla et al., 1996 ). This factor may be affected by crop geometry. Sheath blight epidemics are favoured by close spacing of hills in the case of transplanted rice ( Kannaiyan & Prasad, 1983; Castilla et al., 1996 ), and by high sowing rates in the case of direct-seeded rice ( Mithrasena & Adikari, 1986; Guzman Garcia & Nieto Illidge, 1992). A positive effect of dense canopy on diseases caused by Rhizoctonia solani has also been shown in other crops such as potato ( Firman & Allen, 1995), soybean ( Yang et al., 1990 ) and tall fescue ( Giesler et al., 1996a ). Such canopy–disease relationships reflect the combined effects of microclimate and canopy structure on the spread of this type of disease ( Yang et al., 1990 ; Savary et al., 1995 ; Giesler et al., 1996b ).
The objective of this study was to compare sheath blight epidemics and key epidemiological factors in rice crops with different crop establishment methods and therefore in crop stands that differ in their canopy geometry.
Materials and methods
- Top of page
- Materials and methods
The experiment was conducted in a farmer’s field near Pila, Laguna, Philippines, during the rainy season in 1996 and during the dry season in 1997. IR 72, a semidwarf, high-yielding, short-cycle cultivar, was used in both experiments.
Four crop establishment methods were compared: broadcast direct-seeded rice (DS); and transplanted rice, with spacing across and between rows of 20 × 20 cm (TR20×20), 13 × 25 cm (TR13×25) and 25 × 25 cm (TR25×25).
The experimental design was a randomized complete-block design with four replications. Each individual plot was 3 × 3 m in size, and the distance between plots was 1 m. All observations were taken in the 1 × 1-m square located at the centre of the plots, referred to subsequently as the central area. The area between plots was called the buffer area.
In 1996, direct sowing was carried out on July 19 and transplanting on July 30. In 1997, these operations were conducted on February 5 and February 11, respectively. In the transplanted crop establishment, 10-day-old seedlings were planted at a rate of 7–8 seedlings per hill. In the direct-seeded crop establishment, seeds were soaked overnight in water, incubated for two days in jute bags and broadcast at 90 kg ha−1. A pre-emergence herbicide (Butachlor) was applied before crop establishment in the direct-seeded plots. The entire experimental area was flooded with 5 cm of water above ground from 20 days after crop establishment until 15 days before harvest, when the field was drained. Time was scaled in DAT, the number of days after transplanting in transplanted plots.
The experimental area was irrigated with water flowing from a fish-pond that also provided most of the nitrogen supply required for crop growth. No nitrogen was applied in 1996, and in 1997 urea was applied at 23 and 42 DAT at a rate of 50 kg N/ha.
Rice seedlings were transplanted into the buffer area on the same date as in the transplanted plots, with 20 × 20-cm spacing.
Establishment of sheath blight in individual plots
Rice seedlings were potted at the same time as transplanting into the field, and were grown in a glasshouse. At the tillering stage, they were inoculated by inserting at their base a rice grain/rice hull mixture colonized by mycelium of an R. solani anastomosis group AG1–1A isolate ( Sharma & Teng, 1990; Savary et al., 1995 ). After one week of incubation, the infected plants were used as disease sources. One heavily infected plant was transplanted to the centre of each plot, 35 DAT in 1996, and 38 DAT in 1997. The infection source was removed once sheath blight lesions were observed on the hills or plants surrounding it, which occurred at 7 days after the transplanting of the infected plant in 1996 and 4 days after in 1997.
Assessment and computation of sheath blight incidence and severity
Assessments of sheath blight were done weekly throughout the growing season on hills or plants located in the central area. A tiller was operationally defined as a tiller having at least two fully expanded leaves. The number of tillers m−2 was calculated from counts of tillers in 10 and 20 host aggregation units per plot in the transplanted and direct-seeded stands, respectively. Incidence at the tiller level was defined as: INC = 100 Fh Fit, where Fh is the fraction of infected host aggregation units (hill or plant), and Fit is the fraction of infected tillers in the diseased host aggregation units. Sheath blight severity was defined as: SEV = Sit Fit, where Sit is the average severity in infected tillers.
In the transplanted plots, the fraction of infected hills, Fh, was calculated from the assessment of disease occurrence in all hills standing in the central area. The fraction of infected tillers in the diseased hills, Fit, was computed on the basis of counts of diseased and healthy tillers in ten infected hills selected at random. If fewer than ten diseased hills were observed in a plot, the fraction of diseased tillers was measured in all diseased hills. The average severity in infected tillers, Sit, was computed on the basis of the assessment of sheath blight severity on five infected hills selected from the ten sampled hills. When fewer than five infected hills were observed, severity was assessed on all infected hills. Severity was assessed on one infected tiller per hill as the percentage of total tiller area (stem plus leaves) covered by sheath blight lesions.
In the direct-seeded plots, the fraction of infected plants, Fh, was calculated from the assessment of disease occurrence in all plants standing in one-eighth of the central area. Within this area, the same sampling scheme and computations as in transplanted plots were followed, except that twenty plants were sampled to assess sheath blight incidence, and one tiller per plant in ten infected plants was used to assess sheath blight severity.
Assessment of contact frequency
Assessments of contact frequency were done in all plots of both experiments at the tillering and panicle initiation stages. Counts were made differently depending on the crop establishment method. Contact frequency between hills was measured in transplanted plots, and contact frequency between plants was measured in direct-seeded plots.
In the transplanted plots of the 1996 experiment, two hills per plot were randomly selected within the central area. The total number of leaf-to-leaf and leaf-to-sheath contacts between each sampled hill and all eight hills surrounding it were counted ( Savary et al., 1995 ).
In the transplanted plots of the 1997 experiment, four hills per plot were randomly selected in the central area. In the TR20×20 and TR25×25 treatments, leaf-to-leaf and leaf-to-sheath contacts were counted between each selected hill and two of its neighbours (one on the same row and one along the diagonal). In the TR13×25 treatment, leaf-to-leaf and leaf-to-sheath contacts were counted between each selected hill and three of its neighbours (one on the same row, one on the diagonal and one on the adjacent row, in front of the sampled hill).
In the direct-seeded plots of the 1996 and 1997 experiments, four plants per plot were randomly selected within the central area. The total number of leaf-to-leaf and leaf-to-sheath contacts between the sampled plant and one plant surrounding it were counted.
Computation of contact frequency per square metre
The total number of contacts (sheath-to-leaf plus leaf-to-leaf) per m2 (CF) was estimated as follows, using assessments of contact frequency: if NB is the total number of hills m−2, x is the total number of contacts counted between one hill and its neighbours, and n is the number of neighbours for which counts were made, then:
1 x/n is the number of contacts between one hill and one of its neighbours;
2 8x/n is the number of contacts between one hill and all its neighbours;
3 8x/(2n) is the average number of contacts between one hill and all its neighbours, because contacts occurring between hill A and hill B are the same as those occurring between hill B and hill A. Thus it follows that CF = 8x NB/(2n), and for 1996 (n = 8), CF = x NB/2, for 1997, TR20×20 and TR25×25 plots (n = 2), CF = 2x NB, and for 1997, TR13x25 plots (n = 3), CF = 1·33x NB, with NB = 25, 16 and 30·8 for TR20×20, TR25×25 and TR13×25, respectively.
In the case of direct-seeded rice, a plant was assumed to be surrounded by six plants. Thus: CF = 6x NB /2, where NB is the number of plants m−2 at each assessment of contact frequency.
For each year, the means of the contact frequency over the four replications were compared within each assessment date, according to the Student–Newman–Keuls multiple range test.
Analyses of disease progress curves
The disease progress curves of sheath blight incidence and sheath blight severity were described with the logistic model dy/dt = ay (1 – y), where y is the disease incidence or the disease severity, and a represents the apparent infection rate ( Van der Plank, 1963).
Linear regressions were done using the equation logit(y) = at + b. Separate regressions were done in the two years for the different crop establishment treatments. The parameters estimated from the regressions were compared using a t-test ( Campbell & Madden, 1990).
Two linear regressions were tested using a stepwise, upward procedure in order to synthesize the relationships between disease incidence, disease severity and contact frequency over the two experiments. The regressions have the following shapes:
where CE is a binary variable equal to 0 for direct-seeded stands, and equal to 1 for transplanted stands; CF is the contact frequency at panicle initiation; and INC and SEV are the maximum disease incidence and severity, respectively, observed in each individual plot of both experiments. Thirty-two observations were thus considered in each regression.