The effect of two QTLs for resistance to Meloidogyne incognita in cotton on nematode egression from roots

Authors: DASILVA, MYCHELE - University Of Georgia; Davis, Richard; NICHOLS, ROBERT - Cotton, Inc; Kumar, Pawan; CHEE, PENG - University Of Georgia

Submitted to: Journal of Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2020
Publication Date: 1/21/2021
Citation: Dasilva, M., Davis, R.F., Nichols, R., Kumar, P., Chee, P. 2021. The effect of two QTLs for resistance to Meloidogyne incognita in cotton on nematode egression from roots. Journal of Nematology. 52:e2020-122. https://doi.org/10.21307/jofnem-2020-122.
DOI: https://doi.org/10.21307/jofnem-2020-122

Interpretive Summary: Cotton is widely grown in the southern US and the Southern root-knot nematode (RKN), Meloidogyne incognita, is its most significant pathogen. The germplasm line M-120 RNR is highly resistant to RKN due to two QTLs, qMi-C11 and qMi-C14, that contain resistance genes. Both QTLs reduce total nematode egg production, but the QTLs affect RKN development at different times in the nematode’s life cycle. The QTLs do not appear to affect initial penetration by the nematode, but plants containing qMi-C11 had fewer nematodes in the roots 8 days after inoculation than near isolines without qMi-C11, which may indicate nematode egression from roots. Three greenhouse trials were conducted using cotton isolines to determine whether qMi-C11 and qMi-C14 affect egression of RKN juveniles from roots. Two-week-old seedlings were inoculated with RKN; two days after inoculation (DAI), roots were rinsed and seedlings were transplanted into small cones filled with vermiculite. On each of the five sampling dates (4, 6, 8, 10, and 12 DAI), nematodes that egressed from roots were extracted from the vermiculite and counted and roots were stained to count nematodes that remained in the roots. The effect of resistance QTLs on nematode egression from the roots differed among the trials. Beginning 6 DAI, nematode egression was consistently numerically greater, but inconsistently statistically different, from plants with both QTLs than from plants with neither QTL. Plants with only one QTL generally did not differ from plants with both QTLs, and the effects of qMi-C11 and qMi-C14 did not differ in any consistent way. In a separate experiment at 40 DAI, plants with neither QTL had more eggs per egg mass than did plants with both QTLs, whereas plants with only one QTL had an intermediate number and did not differ from each other. Root gall size was measured in two trials with seedlings in clear plastic bags. Individual galls were labeled when first observed (day 0) and each gall’s size was measured at 0, 7, and 14 days. No consistent differences in gall size were observed. We conclude that 1) qMi-C11 and qMi-C14 do not significantly stimulate nematode egression from cotton roots, 2) both qMi-C11 and qMi-C14 reduce M. incognita eggs/egg mass, and 3) neither qMi-C11 nor qMi-C14 affect gall size.

Technical Abstract: Cotton is widely grown in the southern US and Meloidogyne incognita is its most significant pathogen. The germplasm line M-120 RNR is highly resistant to M. incognita due to two resistance QTLs, qMi-C11 and qMi-C14. Both QTLs reduce total egg production, but the QTLs affect M. incognita development at different times in the nematode’s life cycle. The QTLs do not appear to affect initial penetration of M. incognita but genotypes containing qMi-C11 had fewer nematodes in the roots 8 days after inoculation than near isolines without qMi-C11, which may indicate M. incognita egression from roots. Three greenhouse trials were conducted using cotton isolines to determine whether qMi-C11 and qMi-C14 affect egression of M. incognita juveniles from roots. Two-week-old seedlings were inoculated with M. incognita; two days after inoculation (DAI), roots were rinsed and seedlings were transplanted into small cones filled with vermiculite. On each of the five sampling dates (4, 6, 8, 10, and 12 DAI), nematodes that egressed from roots were extracted from the vermiculite and counted and roots were stained to count nematodes that remained in the roots. The effect of resistance QTLs on M. incognita egression from the roots differed among the trials. Beginning 6 DAI, nematode egression was consistently numerically greater, but inconsistently statistically different, from plants with both QTLs than from plants with neither QTL. Plants with only one QTL generally did not differ from plants with both QTLs, and the effects of qMi-C11 and qMi-C14 did not differ in any consistent way. In a separate experiment at 40 DAI, plants with neither QTL had more eggs per egg mass than did plants with both QTLs, whereas plants with only one QTL had an intermediate number and did not differ from each other. Root gall size was measured in two trials with seedlings in clear plastic bags. Individual galls were labeled when first observed (day 0) and each gall’s size was measured at 0, 7, and 14 days. No consistent differences in gall size were observed. We conclude that 1) qMi-C11 and qMi-C14 do not significantly stimulate nematode egression from cotton roots, 2) both qMi-C11 and qMi-C14 reduce M. incognita eggs/egg mass, and 3) neither qMi-C11 nor qMi-C14 affect gall size.