The effect of pesticides, cultivar selection and soil biofumigation on arbuscular mycorrhizal fungi, using tomato as a model system

The objectives of this study were to determine the effect of three different pesticides, three tomato cultivars, and two mustard biofumigants on the establishment and survival of four species of arbuscular mycorrhizal fungi (Acaulospora capsicula, A. laevis, Funneliformis mosseae and Scutellospora calospora). In vitro spore germination of the four AMF species at standard rates of two fungicides (carbendazim and captan), the insecticide chlorpyrifos, or water (control) was observed weekly for 21 days. After 21 days higher germination was observed for S. calospora spores compared with F. mosseae, A. capsicula and A. laevis, but no differences were observed between the pesticide treatments and the control. The response of the four AMF species to carbendazim applied as a foliar spray, and captan and chlorpyrifos applied as soil drenches, was tested by examining the presence/ absence of colonisation (18 days post application) in tomato roots samples, grown in low P potting mix. Tomato growth responses (shoot height and diameter and root and shoot dry weight) were also measured. Fewer tomato roots treated with carbendazim and captan were colonised compared with chlorpyrifos and non-treatment control. A second experiment observed the response of two AMF species (F. mosseae and S. calospora) to the three pesticides on tomatoes grown in silt loam harvested at 10 and 28 days post application. At the first harvest there were no differences in tomato growth in response to the pesticide treatments, but the plants inoculated with AMF were larger than the non-inoculated controls in all tomato growth measures. Scutellospora calospora root colonisation at the first harvest was negatively affected by carbendazim and chlorpyrifos compared to F. mosseae. At the second harvest growth measures were affected by both AMF species and pesticides, with all showing increased growth in the AMF inoculated tomatoes compared to the control, and most showed increased growth in response to carbendazim and chlorpyrifos compared to captan and the non-treatment control. At the second harvest F. mosseae had fewer colonised roots than S. calospora and the non-inoculated control, and the chlorpyrifos treatment decreased colonisation overall. The effect of the four species of arbuscular mycorrhizal fungi on three cultivars of tomato, Moneymaker and Aunt Ruby’s German Green and Sweet 100 was studied by examining growth responses and the presence/ absence of colonisation of root samples after 6 and 9 weeks. AMF had no effect on tomato growth at 6 weeks, but at 9 weeks shoot dry weights were lower for F. mosseae and S. calospora. Root colonisation in Sweet 100 was higher than Moneymaker and Aunt Ruby’s German Green. Two mustards (Caliente 199 and Brassica juncea) were grown in silt loam for 35 days before being mulched and incorporated into the top 100 mm of silt loam in 4 L containers. AMF spores, sealed in micromesh bags, were buried at within the mulch layer (50 mm deep) and below the mulch layer (150 mm deep) for 14 days before being recovered and tested for in vitro germination, their ability to colonise and effect on the growth of Moneymaker tomatoes. Fewer Acaulospora capsicula spores were recovered in the Caliente 199 and mustard treatments compared with the non-treatment control. Germination of A. capsicula spores recovered from below the mulch was higher than for those recovered from within the mulch, whilst the opposite was seen for A. laevis. Spores buried below the mulch in the mustard treatment produced taller tomato plants than spores in the Caliente 199 and non-treatment control; and spores buried within the mulch had taller plants than the spores buried below the mulch in the Caliente 199 treatment, but no other differences in tomato growth were observed. A higher number of roots were colonised by AMF spores recovered after burial within the mulch compared with below the mulch. This study has demonstrated the resilience of arbuscular mycorrhizal fungi and mycorrhizal associations to horticultural management practices. Neither the pesticides or biofumigant treatments had any permanent effect on the AMF species, nor did the three cultivars show affiliation for a particular AMF species