The preparation method of hydrogels has a significant effect on their structural and physicochemical properties. In this report, physically and chemically cross-linked poly(vinyl alcohol) (PVA) networks containing humic acid (HA) were alternatively prepared by autoclaving (AC) and through glutaraldehyde (GA) addition, respectively, for agricultural purposes. PVA/HA hydrogels were comparatively characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, mechanical assays, scanning electron microscopy, swelling kinetics measurements, and water retention tests in soil. AC hydrogels showed a more homogeneous porous microstructure, higher swelling levels, and a better capacity to preserve the humidity of soil than those obtained by adding GA. Both PVA/HA hydrogels exhibited no phytotoxicity on cultivation trials of Sorghum sp., but the plant growth was promoted with the GA-cross-linked network as compared to the effect of the AC sample. The release behavior of urea was modified according to the preparation method of the PVA/HA hydrogels. After 3 days of sustained urea release, 91% of the fertilizer was delivered from the AC hydrogel, whereas a lower amount of 56% was released for the GA-cross-linked hydrogel. Beyond the advantages of applying PVA/HA hydrogels in the agricultural field, an appropriate method of preparing these materials endows them with specific properties according to the requirements of the target crop.

This study deals with the development of humic acid (HA)-enriched hydrogels aimed at enhancing water retention and promoting plant growth for agricultural applications. A set of hydrogels based on spermidine (SPD)-ionotropically cross-linked gellan gum (GG) were formulated with and without the addition of karaya gum (KG). The hydrogels were structurally characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, mechanical assays, scanning electron microscopy, and swelling kinetic measurements. Water retention tests indicated that both GG/HA and GG/KG/HA hydrogels preserve higher soil moisture levels compared to commercial polyacrylate hydrogels. Biodegradation studies showed that the biopolymer hydrogels lost more than one-third of their weight after 30 days of immersion in a soil aqueous extract. Both GG/HA and GG/KG/HA hydrogels exhibited no phytotoxicity on cultivation trials of Triticum aestivum L. var. Borlaug 100, but the plant growth was promoted with the GG/KG/HA network compared to GG/HA sample. The overall results support the potential use of GG/KG-based hydrogels enriched with HA as sustainable materials for improving soil quality and crop productivity in agricultural systems.

Watermelon is one of the major crops grown in Mexico and represents 4% of the total cultivated area with fruits in this country. In 2013, Sonora State was ranked second in the production of watermelon at a national level. Fungal and oomycete diseases are among the main biotic factors affecting watermelon production, particularly those caused by species of the genera Fusarium, Phytophthora, Pythium, and Rhizoctonia. During the spring of 2013, wilting or death symptoms were confirmed in approximately 50% of ungrafted watermelon plants grown in four sampled fields along the coast of Hermosillo and Guaymas Valley in Sonora, Mexico. On both roots and stems of infected plants, localized lesions were found that were 0.2 to 2.0 cm long, reddish brown, and slightly sunken on the stem base. In some cases, the discolorations encompassed nearly 90% of the root system. One-centimeter pieces from the edge of lesions on stems and roots were superficially disinfected with 1% sodium hypochlorite, then rinsed with sterile distilled water, placed onto petri dishes containing potato dextrose agar (PDA), and incubated at 25°C for 3 days. Fungal colonies were white initially, then turned brown, and septate hyphae were 3.7 to 4.3 μm in diameter and branched at right angles with a constriction at the origin of the branch point. These characteristics are typical of the genus Rhizoctonia. Binucleate cells from five isolates were observed using a lactophenol aniline blue solution stain, according to Ceratobasidium morphological descriptions. Mycelia from five isolates grown on PDA was used for DNA extraction. The rDNA-ITS region was amplified using PCR with the universal fungal primers ITS1 and ITS4 (3). The purified products were separately sequenced in both directions using the same primer pair. The sequences obtained were 99% similar to those of Ceratobasidium sp. AG-F and AG-Fa isolates (accessions KC193238.1 on Tagetes erecta, HQ168370.1 on Musa spp., and JX913821.1 on soy-rice-weeds, respectively) from GenBank (2,4). The pathogenicity of the fungus was tested under growth chamber conditions. Sets of seven healthy watermelon seedlings of the Sugar red variety were inoculated with five isolates of Ceratobasidium. Three disks (8 mm in diameter) of mycelia grown on PDA were placed around the roots of each plantlet. The pots were maintained at 27 ± 0.1°C for 14 days with a photoperiod of 12 h. Seven uninoculated seedlings were used as a control. Initial symptoms showing water-soaked lesions developed on all inoculated seedlings within 6 to 7 days, while typical disease symptoms appeared after 10 to 14 days after inoculation. Seedlings without inoculum were free from infection. The fungus was re-isolated from the inoculated seedlings on PDA, and identified as Ceratobasidium sp., confirmed using morphological characteristics. A similar disease has been reported recently in Italy and Arizona (1); however, this report is the first description of a Ceratobasidium sp. causing root rot of watermelon in Sonora, Mexico. Agricultural areas where the study was carried represent 90% of the total area cultivated with watermelon in this state, so it is necessary to evaluate the impact of this pathogen in the crop. References: (1) C. Nischwitz et al. APS joint meeting, 2013. (2) A. Saroj et al. Plant Dis. 97:1251, 2013. (3) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990. (4) J. Yin et al. Plant Dis. 95:490, 2011.

El estado de Sonora es uno de los principales productores de sandía en México. Cada año, los productores locales enfrentan problemas fitosanitarios, provocados principalmente por hon­gos del suelo. En el presente estudio se analizó la presencia de hongos patógenos asociados con pudrición de raíz en plantas de sandía en las dos zonas de mayor producción en Sonora. El análisis morfológico reveló la presencia de tres géneros de hongos: Fusarium (73%), Ceratobasidium (20%) y Rhizoctonia (6%). Mediante un análisis filogenéti­co multilocus (ITS1, TEF y RPB2 para Fusarium; ITS1 y RPB2 para Rhizoctonia y Ceratobasidium), se identificó a: F. falciforme, F. brachygibbosum y F. oxysporum; además de dos grupos anastomósi­cos de Ceratobasidium sp. (AG-F y AG-A) y dos de Rhizoctonia sp. (AG-4 y AG-6). Aislados repre­sentativos de estas cinco especies causaron pudri­ción de raíz y marchitez de plántulas de sandía a los 21 días después de su inoculación. En este estudio se informa por primera vez de F. falciforme y se define a nivel de grupos anastomósicos las cepas de Rhizoctonia y Ceratobasidium como causantes de pudrición radicular en sandía en la región.

The presence of well-defined and depressed circular leaf spots, dark in their core and surrounded by a light brown dim halo, was observed on Dendrobium nobile plants in the municipality of Ixtaczoquitlán, state of Veracruz, Mexico, in 2019. Colletotrichum isolates were consistently obtained from symptomatic plants of D. nobile. The morphological characteristics of these isolates resembled each other, and their pathogenicity could be confirmed based on the necrotic lesions caused by three of these isolates on D. nobile plants 10 days after inoculation. The causal agent of this disease was identified as Colletotrichum karstii, based on morphological characteristics and the partial sequences of the ITS1-5.8S-ITS2, calmodulin and β-tubulin genes of the DNA of three representative isolates (CKFG004, CKFG005 and CKFG006). To our knowledge, this is the first report which identifies C. karstii as the causal agent of anthracnose of D. nobile leaves in Mexico.