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"Corn stunt" is one of the main corn (Zea mays L.) diseases in the Americas and Dalbulus maidis (DeLong & Wolcott) is the key vector of the pathogen Spiroplasma kunkelii Whitcomb. In Argentina, the corn-producing area is in the temperate region, where vector and pathogen prevalence levels are unknown. In this study, the prevalence and distribution of D. maidis and S. kunkelii in the temperate region of Argentina and D. maidis overwintering ability in this region were determined. Surveys were conducted in 2005-2006 and 2006-2007 seasons to determine D. maidis and S. kunkelii presence, and in winter 2006 to determine the vector overwintering ability. The highest S. kunkelii prevalence and incidence levels were found in the transition area from the temperate to the subtropical region, related to the highest D. maidis prevalence and insects sampled per location. D. maidis adults were found in volunteer corn plants and spontaneous vegetation in autumn and winter months, which were inoculative for the pathogen S. kunkelii. This overwintering ability was related to detection of D. maidis insects in corn crops at early growth stages in the following growing season. This work emphasizes that corn stunt disease is present in the temperate region of Argentina, and this highlights the need to develop proper agronomic practices like monitoring insect vector populations and controlling voluntary plants. This study also indicates that further research is needed to understand the potential yield reduction caused by this pathogen on symptomless plants and population dynamics of the insect vector.

Buffel grass (Cenchrus ciliaris L.) is an important forage grass in arid and semiarid regions. As part of a genetic improvement programme, four genotypes [Biloela (Bl), Americana (Am), Texas (Tx) and Sexual (Sx)] were categorized by tolerance to heat stress in a greenhouse experiment. At 30 d after sowing, half of the plants (control plants) were transferred to growth chambers (28°C day per night), and the other half (pre-treated plants, Prt) were exposed to heat stress treatment (0, 24, 48 and 72 h; 45°C day per night). Malondialdehyde (MDA) content, an indicator of oxidative damage, was determined from foliar samples. During heat stress, Sx showed the earliest increase in MDA (at 24 h) followed by Tx (48 h) and Am and Bl (72 h). Results were compared with heat-stress tolerance evaluated as morphological traits at the end of recuperation (60 d after sowing). Fresh weight and aerial plant height were lowest in the Prt-Sx genotype and highest in Am and Bl genotypes; Tx showed intermediate tolerance. Results suggest that tolerance to heat stress in C. ciliaris genotypes could be related to the capacity for regulating the oxidative damage increase. Foliar MDA content might therefore be used in a genetic improvement programme of C. ciliaris as a potential biochemical indicator for a rapid, simple and low-cost identification of heat-stress tolerant genotypes.

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Buffel grass is a fodder grass that reproduces mainly via apomixis. Using in vitro tissue culture in apomictic species provides alternatives that can be applied in a breeding program. The aims of this work were to establish a protocol for in vitro generation of apomictic buffel grass genotypes using anthers as explants and to evaluate the genetic stability of regenerated plants via flow cytometry (FCM). Three genotypes were able to induce embryogenic calli in Murashige and Skoog (MS) medium supplemented with 6 mg/l of 2,4-dichlorophenoxyacetic acid. Seedling regeneration occurred in a MS medium supplemented with 0.5 mg/l napthaleneacetic acid ? 1 mg/l 6-benzylaminopurin. Seedlings were derived from somatic embryos and the morphogenic process was induced using the somatic tissue of the stamens. Induction and regeneration efficiency depended on the genotype and was affected by date of tiller collection, different pretreatments or the interaction of these variables. FCM analyses in in vitro regenerated plants showed genetic instability in their nuclear DNA content. Plants with lower nuclear DNA content may indicate DNA aneuploids (8.6 %), whereas plants that had twice the value of nuclear DNA content (4.7 %) suggest in vitro polyploidization. This variation observed in apomictic genotypes provides an opportunity to obtain new variants, which may then be included as sources of genetic variability in buffel grass breeding programs.

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