Location: Plant Genetic Resources Conservation Unit
Project Number: 6046-21000-013-043-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Aug 1, 2024
End Date: Jul 31, 2025
Objective:
To evaluate sweetpotato germplasm for resistance against the black rot causing pathogen Ceratocystis fimbriata. We propose to screen the 48 plant introduction (PI) core set, as this set preserves the geographic and phenotypic sweetpotato diversity in the USDA-ARS Plant Genetic Resources Unit. In addition to the 48 core set, we will screen plant introductions with known resistance to the root-knot nematodes Meloidogyne enterolobii and M. incognita, and include two accessions from Japan (Kyuushuu 100 (JP 179349) and Kasho Nourin (JP 168574)) and one breeding line from the Cooperator's sweetpotato breeding program (L16-186) as black rot resistant standards; the total number of accessions to be evaluated is 74 accessions.
Approach:
To identify sweetpotato accessions that prevent symptom development as well as pathogen transmission from roots to slips, the reaction of storage roots and stems will be screened (in separate tests). Storage roots will be wounded with a flamed dissecting needle about 2-3 mm deep into the root at the median. This depth does not penetrate the vascular/cambial ring which may be an important natural barrier to infection and most wound on packing lines are relatively shallow. A 10-µl drop of a suspension of 1 x 106 endoconidia/ml (harvested from a 5- to 7-day-old PDA culture) will be applied to the wound with a micropippetor. The roots will then be cured (85o F, 85-90% RH) for 5 days, and stored at 60-65o F until four weeks after inoculation, when lesion development and lesion length will be assessed.
Stem reactions will be determined using vine cuttings, using only terminal cuttings of 10 to 12 inches long, with the leaves attached. The basal end of five cuttings per PI will be dipped in a fungal suspension of 1 x 106 endoconidia/ml and planted in a six-inch-diameter terra cotta pot filled with 1:1 sand: potting mix. After one month growth in the greenhouse, the vines will be cut off, the lower part of the stem will be removed from the potting mix and washed. The length of lesion from the basal cut will be measured, and since the lesions often stop at a node on the stem, the number of nodes the lesion traversed will also be counted.
In addition to directly inoculating storage roots and stems, the PIs will be grown in the greenhouse in infested sand to identify PIs with black rot resistance in a more natural inoculation system. A fungal suspension of 1 x 106 endoconidia/ml will be added to 1,400 ml autoclaved construction sand, to a final concentration of approximately 1,000 endoconida/ml sand. The infested sand will be thoroughly mixed, by placing it in sealed cans and rotating it for two minutes on a roller table. Unamended autoclaved soil will be used as control. Development of stem lesions will be assessed at harvest, and 3 weeks after harvest lesion development and lesion length will be assessed on storage roots.
All three Ceratocystis black rot screening experiments will be repeated in 2025.
