Etiology Evidence for the Involvement of Soilborne Mites in Pythium Pod Rot of Peanut

SHEW, H. D., and M. K. BEUTE. 1979. Evidence for the involvement of soilborne mites in Pythium pod rot of peanut. Phytopathology 69:204-207. Mites of the genus Caloglyphus (Acarina: Acaridae) were associated with obtained from 90% of the fecal pellets collected from C. micheali after more than 50% of decaying peanut pods collected in a field in which pod rot feeding on mycelial mats of the fungus. P. myriotylum oospores also was caused by Pythium myriotylum. Laboratory cultures of these mites remained viable after passing through the alimentary canal of C. micheali. could be maintained for several months on Pythium aphanidermatum Pythium pod rot was reduced significantly in field and greenhouse tests of growing on potato-dextrose agar slant tubes. In food preference tests several acaricides and broad spectrum insecticides. In greenhouse tests the Caloglyphus micheali was attracted to P. myriotylum; up to 98% of all mites addition of soilborne mites to field soil infested with P. myriotylum responding in food preference tests preferred P. myriotylum over five other significantly increased the incidence of peanut pod rot. fungi isolated from peanut pods. Viable colonies of P. myriotylum were Additional key words: Pythium myriotylum, Caloglyphus spp. Although mites are the most numerous of all soilborne For mite identification, a single gravid female from each mite arthropods, only limited research has been conducted to investigate isolate was transferred to a new culture tube to ensure pure cultures the role of this group of organisms in the initiation of fungal (4,16) of mites for future work. Mites used for identification were and bacterial (6) plant diseases. Several reports of mites as pests of mounted in Hoyer's solution (15). Identification was made by M. peanut (Arachis hypogaea L.) are known. Caloglyphusrodionovi H. Farrier, Entomology Department, North Carolina State Zach. was capable of penetrating the peanut pod and feeding on the University, Raleigh, and by the authors. developing kernels (21). Mites of the genera Caloglyphus and Pod rot control and enhancement. A field test was conducted to Tyrophagus were isolated regularly from subterranean parts of evaluate the effectiveness of several acaricides and broad spectrum peanut plants in South Africa and under certain conditions insecticides in reducing pod rot incidence and severity. The field penetrated pods and increased percent colonization of these pods used had a history of Pythium pod rot. Seeds of peanut cultivar VA by Aspergillusflavus (Link) Fr. (1). 56 R were planted in early May and all standard agronomic Enhancement of peanut pod rot caused by Pythium myriotylum practices were observed throughout the growing season except for Drechs. has been reported in the presence of various soil flora (7,10) application of test insecticides and acaricides. These chemicals were and fauna (9,18) including soil mites (2,3). This study was applied at a rate of 2.24 kg Al/ha as a drench in a 30 cm band over undertaken to determine: (i) which mite species are associated with the row in mid July using approximately 655 L H20/ ha to apply the rotting pods in North Carolina; (ii) if mites enhance Pythium pod chemicals. Pod-rot data were collected the last of September. Plots rot development; and (iii) the possible mechanisms of miteconsisted of two 15.2 m rows and were replicated four times in a associated disease enhancement. randomized complete block design. The soil used in greenhouse tests was Norfolk fine sandy loam MATERIALS AND METHODS taken from the plow layer (top 18 cm) of the field described above. Cultivar NC 5 peanut plants were grown in containers 50 X 35 X 14 Isolates of P. myriotylum and all other fungi used in this study cm. Chemicals were applied as a drench at equivalent rates were isolated originally from peanut pods or plants. Stock cultures described for field tests. After 16 wk plants were rated for pod rot were maintained on glucose yeast-extract agar or potato-dextrose and mite populations were determined using the sugar flotationagar (PDA) at 24-26 C. Soilborne mites were obtained from peanut sieving method (3). pods decayed by P. myriotylum, pods that had overwintered in Pod rotha m t rea field soil, and from soil collected in a field with a history of Pythium plants grown in sterilized fine sandy loam soil in 50 X 35 p 14-cm pod rot. Mites were extracted from soil by a sugar flotation-sieving containersnin tereenhoue at 24-3 CTae in cl technique (3). containers in the greenhouse at 24-30 C. Treatments included Mitechnie (3)e ainfesting soil with (i) P. myriotylum plus mites, (ii) P. myriotylum Mite culture and identification. Following initial isolation of alone, (iii) mites alone, and (iv) no treatment (controls). Fungal mites from peanut pods and soil, mites were placed on agar slant inoculum was introduced by placing five oat grains, inoculated 10 tube cultures of Pythium aphanidermatum (Edson) Fitzp. days prior with P. myriotylum, into each corner of the containers growing on PDA at 24-30 C. New colonies were started by approximately 2.5 cm below the soil surface. Sterile oat grains were transferring 5-10 gravid females to a 5-day-old tube culture of P. substituted in treatments not receiving P. myriotylum. Mites were aphanidermatum. Only adult mites were used in in vitro introduced as atmixtu of isolte oto t ot gra experiments. introduced as a mixture of isolates onto the oat grains. Approximately five living mites were placed in each corner of the container. The soil was infested when the plants were 10 wk of age, 00031-949X/79/000035$03.00/0 the test ran for an additional 10 wk, and the test was repeated three 01979 The American Phytopathological Society times.