No abstract

In 2009, 2010, and 2011, melon plants (Cucumis melo L.) exhibited vine decline in commercial fields in the Municipality of Viesca, State of Coahuila, in the north-central region of Mexico known as La Comarca Lagunera. Symptoms included wilting, leaf yellowing, and vine collapse prior to harvest. Diseased plants showed necrotic root lesions and loss of secondary and tertiary roots. Numerous perithecia containing asci and ascospores typical of Monosporascus cannonballus Pollack & Uecker (3) were found in the root system. M. cannonballus is a typical fungus of hot semiarid climates such as La Comarca Lagunera in which daytime temperatures above 40°C are frequent during the melon growing season. Small root pieces were disinfected with 1.5% sodium hypochlorite for 1 min and plated onto potato dextrose agar (PDA) medium with 0.5 g l–1 streptomycin sulfate and incubated for 7 days at 25°C under dark conditions. The mycelium of the fungus colony was initially white, turning gray about 3 weeks later and yielding black perithecia with one ascospore per asci. The internal transcribed spacer region of ribosomal DNA of isolate 4 was sequenced and submitted to GenBank with Accession No. JQ51935. Pathogenicity of this isolate was confirmed on melon plants (cv. Cruiser) in the greenhouse at 25 to 32°C. Fungus inoculum was produced in a sand-oat hull medium (0.5 l of sand, 45 g of oat hulls, and 100 ml of distilled water), and incubated at 25°C for 50 days (1). Melon seeds were sown in sterile sand in 20-cm diameter and 12-cm depth polyurethane containers, and the inoculum was added to produce a concentration of 20 CFU g–1. Sowing was done in five inoculated containers and thinned to two plants per container, each container representing a replication. Plants were also grown in five noninoculated containers that were used as controls. After 50 days under greenhouse conditions, plants were evaluated for disease symptoms. Melon plants inoculated with M. cannonballus exhibited root necrosis as opposed to healthy roots observed in noninoculated plants. M. cannonballus was reisolated from symptomatic plants, confirming Koch's postulates. M. cannonballus causes root rot and vine decline on melon and has been reported in Brazil, Guatemala, Honduras, India, Iran, Israel, Italy, Japan, Libya, the Netherlands (plants from Russia), Pakistan, Saudi Arabia, Spain, Taiwan, Tunisia, and the United States. M. cannonballus was reported in 1996 in southeastern Mexico in the State of Colima, where watermelon (Citrullus lanatus (Thunb.) Matsum.& Nakai) showed wilting and plant collapse prior to harvest (2). However, to our knowledge, this is the first report of M. cannonballus on melon in Mexico. This is relevant because La Comarca Lagunera region is one of the major melon producing areas in Mexico and M. cannonballus is a pathogen that may cause yield losses of up to 100%. References: (1) B. D. Bruton et al. Plant Dis. 84:907, 2000. (2) R. D. Martyn et al. Plant Dis. 80:1430, 1996. (3) F. G. Pollack and F. A. Uecker. Mycologia 66:346, 1974.

In 2010 and 2011, diseased watermelon plants (Citrillus lanatus [Thunb.] Matsun and Nakai) had chlorotic and wilted leaves and vines prior to harvest in three out of four sampled commercial fields in the Municipality of Matamoros, State of Coahuila, in the north-central region of Mexico known as La Comarca Lagunera. Disease incidence across the fields was 30%. Diseased plants also showed necrotic lesions and loss of secondary and tertiary roots, which can render roots unable to obtain an adequate supply of water and nutrients supporting the aboveground part of the plant before fruit maturity. Roots of affected plants contained perithecia with asci and ascospores typical of Monosporascus cannonballus Pollack & Uecker (4). This fungus has been found in hot semi-arid climates with saline and alkaline soils. Daytime temperatures above 40°C are frequent in north-central Mexico during the watermelon growing season. Small root pieces from 30 plants with disease symptoms (10 plants per field) were taken and surface-sterilized with 1.5% sodium hypochlorite, placed on potato dextrose agar (PDA) medium with 0.5 g/L of streptomycin sulfate at two petri dishes per plant and five root pieces per petri dish, and incubated for 7 days at 25°C in the dark. The fungus was isolated with a frequency of 60%. Mycelia were identified from root tissue based on morphological characteristics. DNA was also extracted in CTAB buffer followed by a phenol/chloroform purification and precipitation in isopropanol and ethanol (2). The internal transcribed spacer region was then amplified from isolate 1 using PCR, sequenced, and submitted to GenBank (Accession Number JQ599552). Pathogenicity of isolates was confirmed on watermelon plants (cv. Sweet summer 800) under greenhouse conditions at 25 to 32°C. Inoculum was produced in a sand-oat hull (Avena sativa) medium (0.5 l of sand, 45 g of oat hulls, and 100 ml of distilled water) and incubated for 50 days (1). Watermelon seeds were sown in sterile sand in 20-cm diameter and 12-cm deep polyurethane containers, where inoculum was added to reach a soil concentration of 20 CFU/g. Four seeds were sown in each of five inoculated containers; plants were thinned to two per container after emergence (each container representing a replication). Similarly, plants were also grown in four noninoculated containers and used as controls. After 50 days, all watermelon plants inoculated with M. cannonballus showed root necrosis in contrast with roots from noninoculated plants. M. cannonballus was reisolated from 80% of inoculated plants, confirming Koch's postulates. M. cannonballus causes severe damage on watermelon and other cucurbits such as cantaloupe (Cucumis melo). This fungus has been reported in the United States, Spain, Tunisia, Libya, Israel, Italy, the Netherlands (plants from Russia), Saudi Arabia, India, Japan, Taiwan, Brazil, Guatemala, and Honduras. To date, M. cannonballus has been reported on watermelon in 1996 in the State of Colima in southeastern Mexico (3). However, to the best of our knowledge, this is the first report of M. cannonballus on watermelon in northern Mexico. References: (1) B. D. Bruton et al. Plant Dis. 84:907, 2000. (2) B. R. Lovic et al. Phytopathology 85:655, 1995. (3) R. D. Martyn et al. Plant Dis. 80:1430, 1996. (4) F. G. Pollack and F. A. Uecker. Mycol. 66:346, 1974.

En el laboratorio, suelo adicionado con glucosa 0.5-4.0 mg g-1 e inundado, el pH y el potencial oxidación-reducción (ORP) alcanzaron valores de 6.2 y -250 mV, también observamos hasta 22 mmol L-1 de ácidos grasos volátiles (VAFs). El sobrenadante del suelo se ajustó o no a pH ~ 4, luego los esclerocios de <em>P. omnivora</em> se sumergieron en él. Los esclerocios murieron solo en sobrenadantes a pH ~4 que provenían del suelo con 2.0 y 4.0 mg g-1 de glucosa añadida. En el campo, hubo seis tratamientos: un testigo o suelo no tratado (C); se agregaron 55 t ha-1 de melaza y el suelo se cubrió con plástico (M); suelo inoculado con <em>Trichoderma harzianum</em> (T); suelo desecado (DS) y sus combinaciones DS+M, DS+T y DS+M+T. Cuando se añadió melaza, el pH y el ORP alcanzaron 6.5 y -200 mV. El pH y el ORP alcanzados en suelos con glucosa y melaza son característicos de la desinfestación reductiva del suelo (RSD). En el campo, los tratamientos se aplicaron al suelo, con cuatro (repeticiones) parcelas por tratamiento y se sembraron 12 nueces por parcela. Después de tres años, no hubo diferencia en la incidencia y la mortandad de los árboles causada por<em> P. omnivora</em>, pero las raíces fueron invadidas por <em>Trichoderma</em> sp.

La necesidad de disminuir el uso de pesticidas sintéticos, bajar costos, incrementar eficiencia para el control de fitopatógenos y de realizar una agricultura orgánica, son razones para mejorar y desarrollar nuevas alternativas de control. La desinfestación biológica (DBS), anaerobia (DAS) o reductiva del suelo (DRS) son sinónimos; en ésta revisión, se utiliza el término DRS y se explica la razón. La DRS constituye un método aplicado al suelo para disminuir o eliminar bacterias, hongos, malezas y nematodos que dañan a los cultivos agrícolas. Esta técnica consiste en agregar una fuente de carbono orgánico de fácil oxidación como residuos de cosecha, semillas, abonos, etc., cubrir con plástico y saturar o inundar el suelo. De esta manera, la fuente de carbono se descompone en condición anaerobia, el suelo se acidifica, el potencial oxido reducción alcanza valores reductivos (-100 a -400 mV) y se generan ácidos grasos volátiles de cadena corta (AGVs). Los AGVs son letales para malezas y fitopatógenos en el suelo. En este ensayo el tema de la DRS, muestra un enfoque histórico, los principios que la sustentan, se hacen propuestas de mejora y evaluación de sus variantes, y se presentan aplicaciones prácticas.