Varietal Descriptors for the Distinction of Underutilized Varieties of Sechium edule (Jacq) Swartz.

Sechium edule (Jacq.) Sw. (Cucurbitaceae) is a species native to Mexico and Central America. The collection, characterization, and evaluation of accessions maintained in genebanks is essential for the conservation of this species. However, there are no specific varietal descriptors that differ from those used in a phenetic approach and are adapted to international registration guidelines to help distinguish, improve, cluster, and protect intraspecific variants of common use and those obtained by breeding. Therefore, 65 morphological descriptors (qualitative and quantitative) were evaluated in 133 accessions obtained from Mexico, Guatemala, and Costa Rica located in the National Germplasm Bank of S. edule in Mexico. These characteristics were observed to be phenetically stable for five generations under the same agroclimatic conditions. In addition, an analysis of amplified fragment length polymorphism (AFLP) was applied to 133 samples from a set of 245 accessions. According to the multivariate analysis, 26 of the 65 descriptors evaluated (qualitative and quantitative) enabled differentiation of varieties of S. edule. The AFLP analysis showed a high level of polymorphism and genetic distance between cultivated accessions and their corresponding wild ancestor. The variations in S. edule suggest that the morphological characteristics have differentiated from an essentially derived initial edible variety (ancestral original variety), but unlike other cucurbits, there is no evidence of the ancestral edible for Sechium since the seed is unorthodox and there are no relicts.

First Report of Fusarium citri Causing Postharvest Fruit Rot of Chayote in Mexico.

In November 2018, symptoms of brown rot were observed on chayote (Sechium edule) var. nigrum spinosum with a 20% disease incidence of 120 harvested fruits in the National Germplasm Bank of Sechium edule, located in the Centro Regional Universitario Oriente (CRUO) from the Chapingo Autonomous University (Huatusco, Veracruz, Mexico). For fungal isolation, pieces from symptomatic fruits were surface disinfected by immersion in a 1.5% NaClO solution for 2 min, rinsed in sterile distilled water, placed in Petri plates containing potato dextrose agar (PDA) amended with kanamycin sulfate, and incubated at 25ºC. Fusarium-like colonies were consistently isolated on PDA and five monoconidial isolates were obtained. A representative isolate was selected for morphological characterization, phylogenetic analysis, and pathogenicity tests. On PDA, colonies exhibited white and fluffy aerial mycelia, with diffused pale brown pigment in the center at 7 days of incubation at 25℃ in darkness. Macroconidia (n= 100) were hyaline, falcate, with 4 to 5 septa, measuring 23.9 to 31.9 × 2.9 to 4.2 µm, and foot-shaped basal cells. Microconidia and chlamydospores were absent. Morphological features were consistent with the description of the Fusarium incarnatum-equiseti species complex (Xia et al. 2019). The isolate was deposited as FUS2 in the Culture Collection of Phytopathogenic Fungi of the Laboratory of Plant Pathology at the Colegio de Postgraduados. For molecular identification, genomic DNA was extracted, and the internal transcribed spacer (ITS) region, partial sequences of translation elongation factor 1-alpha (EF1-α), and the second-largest subunit of RNA polymerase II (rpb2) genes were amplified, and sequenced with the primer sets ITS5/ITS4 (White et al. 1990), EF1-728F/EF1-986R (Carbone and Kohn 1999), and RBP2-5F/RPB2-7R (Liu et al. 1999), respectively. DNA sequences were edited in BioEdit 7.2 and compared with those in the NCBI nucleotide database. Alignments were implemented in MEGA X using reference sequences from Fusarium spp. A phylogenetic tree, including published ITS, EF1-α, and rpb2 sequence data, was constructed for the Fusarium incarnatum-equiseti species complex (FIESC) based on Maximum Likelihood. The sequences were deposited in GenBank (accession nos. ON878083, ON890421, and ON890420). The phylogenetic analysis grouped the isolate FUS2 within the F. citri clade. Pathogenicity of the fungus was verified on 10 healthy chayote fruits var. nigrum spinosum previously disinfested by immersion in a 1% NaClO solution for 3 min and washed in sterile water. A total of 5 mL of a conidial suspension (1 × 106 spores/ml) was sprayed on each whole fruit. Ten control fruit were sprayed with sterile distilled water. The fruits were kept in a moist plastic chamber at 25°C and 12 h light/dark for 30 days. All inoculated fruits developed water-soaked brown lesions (3 to 5 cm in diameter) covered with white mycelium at 15 days after inoculation, whereas no symptoms were observed on the control fruits. The fungus was consistently re-isolated only from the diseased fruits and found to be morphologically identical to the isolate used for inoculation, fulfilling Koch´s postulates. Fusarium citri has been associated with Capsicum sp. and mandarin orange in China, Triticum sp. in Iran, alfalfa in Denmark, and lettuce in the Czech Republic and Italy (Farr and Rossman 2022). To our knowledge, this is the first report of F. citri causing postharvest fruit rot of chayote in Mexico and worldwide.

Use of Gamma Radiation for the Genetic Improvement of Underutilized Plant Varieties.

Agricultural biodiversity includes many species that have biological variants (natives, ecotypes, races, morphotypes). Their use is restricted to local areas because they do not fulfill the commercial requirements; however, it is well documented that these species are a source of metabolites, proteins, enzymes, and genes. Rescuing and harnessing them through traditional genetic breeding is time-consuming and expensive. Inducing mutagenesis may be a short-time option for its genetic improvement. A review of outstanding research was carried out, in order to become familiar with gene breeding using gamma radiation and its relevance to obtain outstanding agronomic characteristics for underutilized species. An approach was made to the global panorama of the application of gamma radiation in different conventional crop species and in vitro cultivated species, in order to obtain secondary metabolites, as well as molecular tools used for mutation screening. The varied effects of gamma radiation are essentially the result of the individual responses and phenotypic plasticity of each organism. However, even implicit chance can be reduced with specific genetic breeding, environmental adaptation, or conservation objectives.

Chayote Fruit (Sechium edule var. virens levis) Development and the Effect of Growth Regulators on Seed Germination.

The chayote fruit is a nontraditional vegetable belonging to the Cucurbitaceae family. The fruit has an endocarpic recalcitrant seed that emerges postharvest, drastically shortening its shelf life. In this study, the changes during fruit and seed development before and after harvest (ah) are reported. Additionally, in order to investigate how growth regulators (GRs) affect seed germination, 2-cloroethylphosphonic acid (CPA) (200 µL L-1), gibberellic acid (GA3) (100 and 200 mg L-1), auxin (2,4-D) (0.5 and 1.0 mM), and abscisic acid (ABA) (0.5 and 1.0 mM) were applied after harvest. The results showed that the chayote fruit reached horticultural maturity at 21 days after anthesis, with a sigmoid trend: phase I featured slow growth and high transpiration; in phase II, growth was accelerated and accumulation of endosperm was observed; and in phase III, both growth rate and transpiration were reduced, soluble sugars increased, and the seed showed 25% cotyledon development. At day 13 ah, CPA, GA3, and 2,4-D (0.5 mM) increased seed germination, with values between 10 and 15 mm of the embryonary axis, and the treatments with 2,4-D (1 mM) and ABA (0.5 and 1.0 mM) retarded their growth (2-6 mm). This research allowed us to reveal the phenological phases and the shelf life of the chayote fruit, as well as the results of possible postharvest treatment with GRs; our results suggest that strategies to delay viviparism and prolong the shelf life of the fruit should be applied before 10 days ah, when the embryonic axis of the seed has not developed.