Susceptibility of Almond (Prunus dulcis) Cultivars to Twig Canker and Shoot Blight Caused by Diaporthe amygdali.

Twenty-five almond cultivars were assessed for susceptibility to Diaporthe amygdali, causal agent of twig canker and shoot blight disease. In laboratory experiments, growing twigs were inoculated with four D. amygdali isolates. Moreover, growing shoots of almond cultivars grafted onto INRA 'GF-677' rootstock were used in 4-year field inoculations with one D. amygdali isolate. In both types of experiments, inoculum consisted of agar plugs with mycelium, which were inserted underneath the bark, and the lesion lengths caused by the fungus were measured. Necrotic lesions were observed in the inoculated almond cultivars in both laboratory and field tests, confirming the susceptibility of all evaluated cultivars to all inoculated isolates of D. amygdali. Cultivars were grouped as susceptible or very susceptible according to a cluster analysis. The relationship between some agronomic traits and cultivar susceptibility was also investigated. Blooming and ripening times were found to be relevant variables explaining cultivar performance related to D. amygdali susceptibility. Late and very late blooming and early and medium ripening cultivars were highly susceptible to D. amygdali. Our results may provide valuable information that could assist in ongoing breeding programs of this crop and in the selection of cultivars for new almond plantations.

Cultivar Susceptibility and Environmental Parameters Affecting Symptom Expression of Red Leaf Blotch of Almond in Spain.

Red leaf blotch (RLB) of almond, caused by Polystigma amygdalinum, is an important foliar disease of this nut tree in the Mediterranean basin and Middle East regions. In recent years, the incidence of this disease has increased in Spain, corresponding to increases in the area of newly planted orchards and the use of susceptible cultivars. In 2009, an experimental orchard including 21 almond cultivars was planted at Les Borges Blanques, Lleida, in northeastern Spain. No fungicide treatments were applied during the 10-year experimental period (2009 to 2018) in order to allow natural disease development. Cultivar susceptibility to RLB was assessed each year, from 2011 to 2018, through visual observations of symptoms in naturally infected trees. The experimental results led us to classify the cultivars into five susceptibility groups. The most susceptible were Tarraco, Guara, Tuono, Marinada, Desmayo Largueta, and Soleta, whereas Mardía was the most tolerant. The annual incidence of disease was positively correlated with accumulated rainfall in spring, and especially in April, while it was negatively correlated with high spring and summer temperatures, especially in May. These findings could be used to improve disease management strategies by identifying the most susceptible cultivars and improving the timing of fungicide application.

Remote Sensing Energy Balance Model for the Assessment of Crop Evapotranspiration and Water Status in an Almond Rootstock Collection.

One of the objectives of many studies conducted by breeding programs is to characterize and select rootstocks well-adapted to drought conditions. In recent years, field high-throughput phenotyping methods have been developed to characterize plant traits and to identify the most water use efficient varieties and rootstocks. However, none of these studies have been able to quantify the behavior of crop evapotranspiration in almond rootstocks under different water regimes. In this study, remote sensing phenotyping methods were used to assess the evapotranspiration of almond cv. "Marinada" grafted onto a rootstock collection. In particular, the two-source energy balance and Shuttleworth and Wallace models were used to, respectively, estimate the actual and potential evapotranspiration of almonds grafted onto 10 rootstock under three different irrigation treatments. For this purpose, three flights were conducted during the 2018 and 2019 growing seasons with an aircraft equipped with a thermal and multispectral camera. Stem water potential (Ψ s t e m ) was also measured concomitant to image acquisition. Biophysical traits of the vegetation were firstly assessed through photogrammetry techniques, spectral vegetation indices and the radiative transfer model PROSAIL. The estimates of canopy height, leaf area index and daily fraction of intercepted radiation had root mean square errors of 0.57 m, 0.24 m m-1 and 0.07%, respectively. Findings of this study showed significant differences between rootstocks in all of the evaluated parameters. Cadaman® and Garnem® had the highest canopy vigor traits, evapotranspiration, Ψ s t e m and kernel yield. In contrast, Rootpac® 20 and Rootpac® R had the lowest values of the same parameters, suggesting that this was due to an incompatibility between plum-almond species or to a lower water absorption capability of the rooting system. Among the rootstocks with medium canopy vigor, Adesoto and IRTA 1 had a lower evapotranspiration than Rootpac® 40 and Ishtara®. Water productivity (WP) (kg kernel/mm water evapotranspired) tended to decrease with Ψ s t e m , mainly in 2018. Cadaman® and Garnem® had the highest WP, followed by INRA GF-677, IRTA 1, IRTA 2, and Rootpac® 40. Despite the low Ψ s t e m of Rootpac® R, the WP of this rootstock was also high.