Resistance to rust (Puccinia arachidis Speg.) identified in nascent allotetraploids cross-compatible with cultivated peanut (Arachis hypogaea L.)

Peanut rust, caused by Puccinia arachidis Speg., is a foliar disease that plagues peanut production along with early and late leaf spots, Passalora arachidicola (Hori) U. Braun and Nothopassalora personata (Berk. & M.A. Curtis) U. Braun, C. Nakash, Videira & Crous, respectively. Rust can cause up to 80% yield losses without control and is widespread in tropical countries but is also a sporadic problem in the United States. An integrative plant management strategy with rust resistant peanut cultivars is needed to decrease dependence on costly fungicides and increase yields for farmers who cannot afford or do not have access to fungicides. Only moderate levels of rust resistance have been found in cultivated peanut germplasm, but fortunately, high resistance to rust has been identified in wild Arachis species that can be introgressed into peanut cultivars. In this study, 16 diploid, wild Arachis species, five diploid, interspecific hybrids, 11 unique, allotetraploid interspecific hybrids, and two cultivated peanut controls were tested for resistance to rust. Resistance was evaluated in vitro by incubation time, susceptibility index (calculated based on the number of lesions of different diameters)/ leaf area, total number of lesions/ leaf area, and total number of sporulating lesions/ leaf area. All wild Arachis species tested were very highly resistant to rust, except for A. ipaënsis , the B-genome progenitor of cultivated peanut. Additionally, all interspecific hybrids and synthetic allotetraploids not produced with A. ipaënsis as a parent did not show symptoms for rust. Any of these nine synthetic allotetraploids, BatCor , BatDur 1, BatDur 2, BatSten 1, GregSten , MagCard , MagDio , MagDur , and ValSten 1 are recommended for progression to QTL mapping of rust resistance. These resistance QTLs can be pyramided into peanut cultivars to protect yields in the United States and to increase yields in tropical, developing countries for farmers that cannot afford, or do not have access to, costly fungicides.

Evaluation of Leaf Spot Resistance in Wild Arachis Species of Section Arachis

Wild diploid Arachis species are potential sources of resistance to early (ELS) and late (LLS) leaf spot diseases caused by Passalora arachidicola (syn. Cercospora arachidicola Hori), and Nothopassalora personata (syn. Cercosporidium personatum (Berk. & Curt.) Deighton), respectively. Within section Arachis, limited information is available on the extent of genetic variation for resistance to these fungal pathogens. A collection of 78 accessions representing 15 wild species of Arachis section Arachis from the U.S peanut germplasm collection was evaluated for resistance to leaf spots. Screening was conducted under field (natural inoculum) conditions in Dawson, Georgia, during 2017 and 2018. Accessions differed significantly (P < 0.01) for all three disease variables evaluated, which included final defoliation rating, ELS lesion counts, and LLS lesion counts. Relatively high levels of resistance were identified for both diseases, with LLS being the predominant pathogen during the two years of evaluation. This research documents new sources of resistance to leaf spot diseases selected from an environment with high inoculum pressure. The presence of ELS and LLS enabled the selection of resistant germplasm for further introgression and pre-breeding.

A Note on a Greenhouse Evaluation of Wild Arachis Species for Resistance to Athelia rolfsii

Athelia rolfsii (Curzi) C.C. Tu & Kimbr. is the one of the most damaging pathogens of cultivated peanut, causing the soilborne disease known regionally as white mold, stem rot, or southern blight. Because the genetic base for cultivated peanut is narrow, wild Arachis species may possess novel sources of disease resistance. We evaluated 18 accessions representing 15 Arachis species ( batizocoi , benensis , cardenasii , correntina , cruziana , diogoi , duranensis , herzogii , hoehnei , kempff - mercadoi , kuhlmannii , microsperma , monticola , simpsonii , williamsii ) in the greenhouse for resistance to At. rolfsii . Assays were conducted on intact plants propagated from rooted cuttings inoculated with mycelial plugs, and lesion length and mycelial growth were measured at 4, 6, 10, and 12 days after inoculation. For lesion length, Arachis batizocoi (PI 468326 and PI 468327), and A. kuhlmannii PI 468159 were the most susceptible entries with a mean lesion length >50 mm at 12 days after inoculation. Arachis microsperma (PI 666096 and PI 674407) and A. diogoi PI 468354 had the shortest lesions with mean lengths ≤16 mm at 12 days after inoculation. Arachis cruziana PI 476003 and the two A. batizocoi PIs had the highest mean area under the disease progress curves (AUDPCs), and the lowest AUDPC was obtained from the A. microsperma PI 674407. Mycelial growth was correlated with lesion length in most species except A. monticola PI 497260 . These results may be useful to peanut geneticists seeking additional sources of resistance to Athelia rolfsii .

Growth Chamber Assay for Evaluating Resistance to Athelia rolfsii

ABSTRACT Planting resistant cultivars is most sustainable method for managing Athelia rolfsii (= Sclerotium rolfsii), one of the most damaging pathogens of peanut worldwide. However, evaluating germplasm for resistance in the field can be complicated by unfavorable environmental conditions, uneven distribution of sclerotia in soil, and difficulty in growing non-standard peanut genotypes such as wild species. Thus, a growth-chamber assay was used to screen for resistance to A. rolfsii in the laboratory. Thirteen peanut genotypes were used to test the assay: cultivars Georgia-03L, Georgia-12Y, Florida-07, Georgia-07W, Tamrun OL02, FloRun ‘107′, Georgia-06G, and U.S. mini-core accessions CC038 (PI 493581), CC041 (PI 493631), CC068 (PI 493880), CC384 (PI 155107), CC650 (PI 478819), and CC787 (PI 429420). Lesion length, as well as length of visible mycelium, on the main stem and a side stem were recorded at 4, 7, 10, and 13 days after inoculation. In general, patterns of lesion and mycelium growth were similar. The most resistant genotypes, Georgia-03L and CC650, had the smallest lesions and least mycelium growth. However, Georgia-12Y, one of the most resistant cultivars available today, appeared less resistant than Georgia-03L in the assay. Other commercial cultivars were intermediate in lesion and mycelium lengths. The most susceptible entries were CC038, CC041, and CC787. Despite limitations in discriminating among genotypes with intermediate resistance to A. rolfsii, these assays may be useful for pre-screening germplasm to identify physiologically resistant and highly susceptible entries, as well as for screening Arachis species that are difficult to grow in the field.