PEST MANAGEMENT: DISEASES Managing Seedling Disease Problems on Rice Through Fungicides, Adapted Cultivars, and Cropping Systems C.S. Rothrock, R.L. Sealy, F.N. Lee, M.M. Anders, and R. Cartwright ABSTRACT in Arkansas rice fields. In 2002, many fields in Arkansas had severe stand losses. Three control practices are being examined in this project for managing stand losses due to seedling diseases: 1) seed treatment fungicides, 2) development of cold-tolerant cultivars, and 3) the use of disease-suppressive production systems. The three types of fields from which seedlings were taken in 2002 were: a) fields at the Rice Research and Extension Center (RREC) at Stuttgart where cold-tolerant and standard genotypes of rice were planted over a range of soil environments; b) fields from several rice-producing counties in Arkansas with the stand-establishment problems; and c) fields at the RREC where there is a continuing study of the impact of various cultural practices crop rotation and tillage on rice yields. From the pathogenicity studies outlined, results suggest that isolates of Pythium spp. from fields having poor stand establishment in 2002 were more virulent than isolates from randomly selected plants from field studies, supporting the diagnosis that Pythium seed and root rot, caused by Pythium spp., was the cause of these stand failures. These Pythium isolates will be used to determine if the cold-tolerant rice genotypes have increased resistance to Pythium damping-off at lower planting temperatures compared to standard cultivars. INTRODUCTION in Arkansas rice fields. Previous research, funded by the Rice Research and Promotion 237
AAES Research Series 504 Board, identified the role of environmental factors and soilborne plant pathogens in limiting rice stand establishment with the goal of determining the conditions where soilborne pathogens play an important role in stand establishment and developing control strategies for reducing seedling disease losses. Pythium spp. significantly reduced stands and growth of rice, especially at cooler temperatures. As growers look to planting earlier, they increase the risk of cool conditions and rice will have to overcome low soil temperatures and also the increased activity of Pythium spp. These studies also demonstrated that Pythium seedling disease decreases the development of plants over a greater range of temperatures than found for stand losses. Three control practices are being examined in this project; seed treatment fungicides, development of cold-tolerant cultivars, and use of disease suppressive production systems to manage losses from seedling diseases. PROCEDURES Seedlings were sampled from three types of fields in the attempt to isolate pathogenic soilborne fungi from these roots, in particular Pythium spp., in 2002. The three types of fields from which these seedlings were taken were: a) fields at the RREC where cold-tolerant genotypes and standard genotypes of rice were planted at three planting dates, beginning in mid-february 2002 (seedlings were harvested at approximately 4 weeks after planting); b) fields from several rice-producing counties in Arkansas that were experiencing problems with the establishment of successful and uniform stands; and c) fields at the RREC where there is a continuing study of the impact of various cultural practices crop rotation and tillage on disease severity and yield in rice. About 2300 root systems of seedlings were processed and plated on Pythium-selective medium, P 5 ARP, from these studies. Isolates were grouped according to cultural characteristics, including growth rate, on several types of media. Representative isolates from each cultural group were then tested for virulence at two different temperature regimes. The first regime was ambient temperature in the greenhouse during the summer of 2002. Here and in all studies, rice seeds were planted at the rate of 10 seeds per cup in sterile vermiculite in Styrofoam cups. As expected, the temperatures in the greenhouse at this time of year were quite elevated, ranging from an average low of 18.5 C (65 F) to an average high of 27.3 C (81 F).The second temperature regime was a constant temperature of 15.5 C (60 F) using growth chambers. RESULTS AND DISCUSSION Hundreds of isolates of Pythium spp. were recovered. These isolates were grouped according to cultural characteristics, including growth rate, on several types of media into 92 cultural groups. In the greenhouse, 65 to 81 F, only 9 of the 92 isolates tested, or ~10%, were even moderately virulent (i.e., 60% emergence). Of these virulent isolates, only 9% of isolates from the cool-temperature germination study fields 238
B.R. Wells Rice Research Studies 2002 and 6% of those from the cropping-system study were virulent, but 13% of isolates from rice-production fields with stand-establishment problems were virulent. As might be expected with Pythium spp., the number of isolates exhibiting moderate to severe virulence (i.e., 40% emergence) increased tremendously at the cooler temperature in the growth chamber. At 60 F, which is at the low range for rice germination, of the 92 isolates tested, 39 were moderately to severely virulent. Again, there was a much larger percentage of virulent isolates from rice-production fields with stand-establishment problems (82% were virulent) than from either the cool-temperature germination study fields (26% virulent) or the cropping-system study (41% virulent). It is expected that more isolates of virulent soilborne pathogens would be found in production fields with stand-establishment problems than in experimental fields without such problems. These results indicate that the stand-establishment problems in rice production fields have a biotic component; specifically, a component relating to seed rot, damping-off, and root rot from Pythium spp. in these fields. Grower Fields In 2002, many fields in Arkansas had severe stand losses. From samples provided by Dr. Rick Cartwright, Pythium spp. were the primary pathogens isolated. From the controlled studies outlined, results suggest that isolates from fields having stand problems in 2002 were more virulent than isolates from randomly selected plants from field studies in 2002, supporting the diagnosis that Pythium spp. were the cause of these stand failures. Cold Tolerance Certain genotypes of rice have recently been found to have the ability to emerge at soil temperatures that are significantly lower than those at which standard cultivars of rice can begin the process of germination and emergence. Planting at lower soil temperature may increase Pythium damage. Similarly, good stand establishment under marginal soil temperatures might suggest these genotypes have some Pythium resistance. The role of cold-tolerant genotypes for the control of Pythium seedling disease was examined by sampling seedlings from the nurseries established by Dr. Fleet Lee. Isolation of Pythium spp. from different genotypes did not show that cold-tolerant or -sensitive lines differed much in recovery of Pythium (Table 1). Seed treatment fungicides did reduce Pythium colonization (Table 1). Cropping System The importance of cropping systems on seedling diseases was examined in 2002 in the cropping-system study of Dr. Merle Anders. Large increases in growth were documented early in the season when soilborne pathogens were eliminated by fumigation. However, the role of Pythium spp. in these growth increases from soil fumigation 239
AAES Research Series 504 is still unclear. It was found that colonization of rice seedlings by Pythium spp. was greater in non-tilled than tilled plots. Also, root weight was greater for seedlings in a rice-soybean rotation than in continuous rice. These differences between crop rotations were eliminated by fumigation, suggesting that differences are related to disease. In the coming year, fungicides with activity against only Pythium spp. will be used to try to reproduce these results. Future Research After examining virulence under three temperature regimes, we will choose appropriate Pythium isolates and challenge seedlings of the cold-tolerant rice genotypes and those of standard rice cultivars with these isolates under a high- and low-temperature regime in growth chambers. Our objective here will be to see if the genetic capacity for germination at low temperatures for these genotypes is related to an increased resistance to Pythium damping-off at lower planting temperatures. Studies will be conducted by planting genotypes in soil not infested or infested with Pythium spp. SIGNIFICANCE OF FINDINGS in Arkansas rice fields. In 2002, many fields in Arkansas had severe stand losses. Three control practices are being examined in this project; seed treatment fungicides, development of cold-tolerant cultivars, and the use of disease-suppressive production systems. The three types of fields from which seedlings were taken in 2002 were: a) fields at the RREC where cold-tolerant genotypes and standard genotypes of rice were planted at three planting dates, beginning in mid-february, 2002; b) fields from several riceproducing counties in Arkansas that were experiencing problems with the establishment of successful and uniform stands; and c) fields at the RREC where there is a continuing study of the impact of various cultural practices crop rotation and tillage on disease severity and yield in rice. From the pathogenicity studies outlined, results suggest that isolates of Pythium spp. from fields having poor stand establishment in 2002 were more virulent than isolates from randomly selected plants from field studies in 2002, supporting the diagnosis that Pythium spp. were the cause of these stand failures. Pythium isolates will be used to determine if the cold-tolerant rice genotypes have increased resistance to Pythium damping-off at lower planting temperatures compared to standard cultivars. ACKNOWLEDGMENTS This research was conducted with the support of the Rice Research and Promotion Board. 240
B.R. Wells Rice Research Studies 2002 Treatment Table 1. Relationship of fungicides and cold tolerance to Pythium recovery from rice (3 rd planting date). Pedigree PI560265 (cold-tolerant) Kaybonnet (cold-tolerant) Lemont (cold-sensitive) PI560235 (least disease-susceptible) Fungicide Seed treatment None Pythium recovery (%) 68 a 73 a 68 a 55 a 48 b 84 a 241