2012. Plant Management Network. This article is in the public domain.
Effects of Soybean Leaf and Plant Age on Susceptibility to Initiation of Infection by Phakopsora pachyrhizi
M. R. Bonde, S. E. Nester, and D. K. Berner, Foreign Disease-Weed Science Research Unit, USDA-ARS, Fort Detrick, MD 21702
Bonde, M. R., Nester, S. E., and Berner, D. K. 2012. Effects of soybean leaf and plant age on susceptibility to initiation of infection by Phakopsora pachyrhizi. Online. Plant Health Progress doi:10.1094/PHP-2012-0227-01-RS.
To help resolve the question of relationship of soybean leaf and plant age to susceptibility to infection by Phakopsora pachyrhizi, we inoculated 50-day-old plants of cv. Williams 82 with a suspension at 3 × 10³ urediniospores/ml, placed in growth chambers under a range of temperature conditions, and later examined for number of lesions. In addition, plants ranging in age from 20 days to 79 days were inoculated, placed in a greenhouse, and examined by leaf for number of lesions. Results showed that Williams 82 did not vary in susceptibility among leaves at different positions regardless of temperature, or among leaves at different positions (up to 11 trifoliolates) on individual plants ranging in age from 20 to 79 days at inoculation. However, on a per-plant basis, there was a gradual decrease in numbers of lesions as plants increased in age at inoculation. For example, whereas 37 lesions/cm² leaf area were produced on 20-day-old plants, only 10 were produced on 79-day-old plants. Although the study does not rule out the possibility that there might be cultivar dependent differences in age response, it does strengthen the possibility that older plants are less susceptible, and that the increase in disease observed in the field during fall months may be due to an increase in favorability of the environment for disease or increase in inoculum pressure.
Soybean rust, caused by Phakopsora pachyrhizi Syd., was discovered in Japan in 1902 (9). Since then, the disease has spread to Australia (6), Africa (11), and South America (15,19). In November 2004, discovery of soybean rust in the southeastern United States (20) generated considerable concern about a possible threat to the United States soybean industry. In many parts of the world the disease has caused significant yield loss, becoming a major constraint to soybean production (6). The pathogen is capable of overwintering on kudzu (4,10) and field studies have shown that leaf surface wetness impacts disease severity (6,17).
Disease resistance in many plant species is affected by plant growth stage (1,7,14 ,21 ,23,25). For example, some research suggests that resistance in potato (Solanum tuberosum L.) to late blight [Phytophthora infestans (Mont.) de Bary], is highest when the plants begin to flower, and that plants younger or older than this stage were more susceptible (7). Other research, however, suggests that young, pre-flowering potato plants that were susceptible to late blight became even more susceptible as the plants entered the flowering stage (7). Disease severity of soybean rust has been reported to increase toward the end of the growing season as plants mature (12,22,27). Some researchers have attributed increased susceptibility to plant maturity. For example, Young et al. (27) reported in northern Florida that soybean plants at growth stages R4 and R5 were more susceptible to soybean rust than younger plants. In Taiwan, Tschanz (22) reported that rapid increases in total lesion numbers under field conditions occurred after the R1 growth stage, beginning flowering, even though abundant inoculum was available throughout the growing season.
Contrary to these findings are reports that soybean plants of all stages are susceptible, and that young plants either are as susceptible or more susceptible than older plants (12,16). For example, Melching et al. (12), conducting research in a plant disease containment facility in Frederick, MD, prior to the entry of the pathogen into the United States, reported that both young soybean plants and individual leaves on plants were more susceptible than older plants and leaves. Moreira et al. (16) reported in Brazil that they observed no increase in soybean rust related to plant development. The issue of plant and/or leaf age and susceptibility to infection by P. pachyrhizi is unresolved. A determination of these relationships would significantly strengthen soybean rust epidemic prediction models.
The objective of this study was to evaluate the effects of plant and leaf age of a representative United States soybean cultivar, Williams 82, on susceptibility to infection from urediniospores of P. pachyrhizi. Williams 82 is a late group III, relative maturity 3.9, indeterminate variety (2), and was selected for this study because it serves as a standard cultivar used in epidemiological studies at FDWSRU (3,5). An understanding of how it reacts to environment at all growth stages would help in interpreting results from other experiments. Results from the study also should help determine dynamics of soybean rust development in the field and provide knowledge useful for improved soybean rust models and prediction of epidemics.
Leaf Age Study
The first set of experiments was conducted to determine the effect of leaf age on 50-day-old plants under a range of temperature conditions. In order to lessen the impact of environment, 50-day-old soybean plants had all leaves removed except the 6th through 9th trifoliolate leaves. Each remaining leaf was inoculated by dipping into an aqueous suspension of 3 × 10³ urediniospores/ml of 0.02% Tween 20. Plants then were incubated 16 h in a dew chamber at 20°C and then randomly distributed among three temperature-controlled growth chambers (specific temperature conditions depending on the particular experiment), three plants per chamber.
The experiment was conducted nine times using an experimental design in which inoculated plants were incubated under a range of temperatures with peak daily temperatures ranging from 17 to 41°C. Chambers were programmed to operate with diurnal temperature cycles characteristic of the field during the growing season in the southeastern United States. Experiments were conducted in sets of three, each set including one temperature pattern included within a different set of experiments. This allowed an overlap of temperature conditions, via temperature patterns, throughout the study and thus facilitated comparison of results. For each temperature pattern, the difference between the daily high and low was 13°C. The temperature patterns, designated by their respective peak temperatures (spaced 4°C apart) were 17°C, 21°C, 25°C, 29°C, 33°C, 37°C, and 41°C (Table 1), and day length 14 h.
At 33 days following inoculation, plants were removed from growth chambers and inoculated leaves excised and photographed for later determination of lesion densities. The data for average number of lesions/cm² leaf area were combined for each leaf position for the three replicate plants within each growth chamber, and data for each temperature profile were combined for all experiments.
Table 1. Lesions/cm² leaf area on trifoliolates of inoculated plants incubated at specific temperature profiles designated by the maximum diurnal temperature
* Leaf position was not significant (P = 0.699) based on analysis of variance.
Data for numbers of lesions/cm² leaf area for each leaf position and temperature profile were analyzed as a mixed model using the MIXED procedure of Statistical Analysis System (SAS) software version 9.2 (SAS Institute Inc., Cary, NC). Temperature profile and trifoliolate position were included as fixed effects in the model while experiment (run) was included as a random effect. Least squares means, standard errors, and P > |t| values in t-tests against zero were generated for temperature profiles and trifoliolate position.
Although with some plants there were more lesions on either younger or older leaves, differences were not significant (Table 1). Least squares mean estimates of all temperature profiles, except 37°C and 41°C, were significantly greater than zero. Measurements of leaf areas showed that during the period from inoculation to data collection there was no leaf expansion which potentially could complicate data interpretation. The conclusion was that leaf position had no effect on infection, regardless of temperature.
Simultaneous Plant and Leaf Age Study
The second set of experiments was conducted to determine both effects of leaf position and plant age on susceptibility to infection. On plants ranging in age from 20 to 79 days at inoculation, each leaf up to the 11th trifoliolate, if present, was inoculated as previously described, plants incubated in a dew chamber, and placed on a greenhouse bench at 20 to 25°C for disease development.
Fourteen days later, inoculated leaves were removed, photographed, and lesion numbers determined. The experiment was conducted four times, and each repeat of the experiment was treated as a random replication effect in the analysis. The numbers of lesions were compared among leaf positions as above and found not to be significantly different.
The numbers of lesions/cm² leaf area for each leaf on a plant were averaged to determine the mean numbers of lesions/cm² leaf area for the plant. Average numbers of lesions/cm² leaf area for each plant of each plant age were analyzed as a non-linear mixed model using the NLMIXED procedure of SAS. The data were modeled to an exponential decline with plant age as the independent variable. Experiment was included as a random effect in the model. The model was: lesions per cm² = b1*exp(b2*age) + u, where u was the random experiment effect. Both the dependent variable and u were considered normally distributed variables with mean of 0 and variances of s²e and s²u, respectively. All parameters were estimated using the adaptive Gaussian quadrature method of NLMIXED.
Age of plants had a significant effect on numbers of lesions/cm² leaf area (Fig. 1). Estimates for both of the parameters b1 and b2 were significantly (P < 0.05) different than zero. The average number of lesions/cm² leaf area was 10 for plants 79 days old, 12 for plants 65 days old, 19 for plants 50 days old, 26 for plants 35 days old, and 37 for plants 20 days old at the time of inoculation. The numbers of lesions/cm² leaf area declined progressively with increasing plant age (Fig. 1).
Leaf position at the time of inoculation had no effect on numbers of lesions which subsequently developed. Plant age did have an effect on susceptibility to infection. In all four experiments, younger plants developed greater disease than older plants (Fig. 1). This was similar to Melching et al. (12), however they did not include plants in the reproductive stages. We used a single, MG III indeterminate cultivar, Williams 82, suited for the mid-western states. MG IV through VIII cultivars are grown in the southern states, and often are determinate (28). It is possible that susceptibility to soybean rust is cultivar dependent. Additional cultivars and rust isolates should be compared and include a range of both determinate and indeterminate cultivars.
Recently, Dias et al. (8) reported that the amount of shading of soybean plants in the field affected severity of soybean rust. As the extent and duration of shading increased, so did the severity of disease. They suggested that shorter days were partly responsible for the higher levels of disease during fall months, and that the greater levels of disease deep within the canopy were due to shading. Other potential factors that could cause increase of disease during fall include lower temperatures and increased moisture. A preliminary study from our laboratory has shown that when the daily peak temperature consistently was at or above 33°C, development of soybean rust was significantly reduced (3).
The development of soybean rust in the field is controlled by many factors that we do not completely understand. The research presented here deals with the relationship of soybean leaf and plant age to susceptibility and infection. As more knowledge is gained, soybean rust models will become valuable in controlling this important disease.
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