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S.I. Torri , C. Descalzi, and E. Frusso. 2009. Estimation of leaf area in pecan cultivars (Carya illinoinensis). Cien. Inv. Agr. 36(1): 53-58. Nondestructive and mathematical approaches of modeling can be very convenient and useful for plant growth estimation. The objective of this research was to develop a simple, accurate and nondestructive predictive model for leaf area (LA) estimation in different pecan cultivars (Carya illinoinensis) commonly found in Argentina. Linear and exponential regression equations were fi tted and evaluated for pecan cultivars 'Desirable', 'Harry Super', 'Kernodle', 'Mahan', 'Mahan-Stuart', 'Shoshoni', 'Stuart' and 'Success' (grafted on seedling pecan rootstocks, 24 years old, planted at 15 x 15 m) using length (L) and width (W) alternatively or both L and W in a stepwise analysis as nonforced independent variables. Stepwise regression analysis using L and W variables for each cultivar fi tted the data better than L or W alone. A general equation for all cultivars showed high accuracy (R 2 = 0.93, p<0.0001). However, for certain cultivars, the general equation provided lower precision than the model for each cultivar. Therefore, we conclude that LA can be estimated through the general or individual model for each cultivar.

The genetic diversity of 14 Japanese plum (Prunus salicina Lindl) landraces adapted to an ecosystem of alternating flooding and dry conditions was characterized using neutral simple sequence repeat (SSR) markers. Twelve SSRs located in six chromosomes of the Prunus persica reference genome resulted to be polymorphic, thus allowing identification of all the evaluated landraces. Differentiation between individuals was moderate to high (average shared allele distance (DAS) = 0.64), whereas the genetic diversity was high (average indices polymorphism information content (PIC) = 0.62, observed heterozygosity (Ho) = 0.51, unbiased expected heterozygosity (uHe) = 0.70). Clustering and genetic structure approaches grouped all individuals into two major groups that correlated with flesh color. This finding suggests that the intuitive breeding practices of growers tended to select plum trees according to specific phenotypic traits. These neutral markers were adequate for population genetic studies and cultivar identification. Furthermore, we assessed the SSR flanking genome regions (25 kb) in silico to search for candidate genes related to stress resistance or associated with other agronomic traits of interest. Interestingly, at least 26 of the 118 detected genes seem to be related to fruit quality, plant development, and stress resistance. This study suggests that the molecular characterization of specific landraces of Japanese plum that have been adapted to extreme agroecosystems is a useful approach to localize candidate genes which are potentially interesting for breeding.

Scab, caused by the plant pathogenic fungus Venturia effusa, is a major disease of pecan in South America, resulting in loss of quantity and quality of nut yield. Characteristics of the populations of V. effusa in South America are unknown. We used microsatellites to describe the genetic diversity and population structure of V. effusa in South America, and determined the mating type status of the pathogen. The four hierarchically sampled orchard populations from Argentina (AR), Brazil (BRC and BRS) and Uruguay (UR) had moderate to high genotypic and gene diversity. There was evidence of population differentiation (Fst = 0.196), but the correlation between geographic distance and genetic distance was not statistically significant. Genetic differentiation was minimal between the UR, BRC and BRS populations, and these populations were more clearly differentiated from the AR population. The MAT1-1 and MAT1-2 mating types occurred in all four orchards, their frequencies did not deviate from the 1:1 ratio expected under random mating, but multilocus linkage equilibrium was rejected in three of the four populations. The population genetics of South America populations of V. effusa has many similarities to the population genetics of V. effusa previously described in the U.S.A. Characterizing the populations genetics and reproductive systems of V. effusa are important to establish the evolutionary potential of the pathogen, and thus its adaptability – and can provide a basis for informed approaches to utilizing available host resistance and determining phytosanitary needs.

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