Dani Shtienberg’s research while affiliated with Agricultural Research Organization ARO and other places

Olive leaf spot, also called peacock eye disease, is caused by the hemibiotrophic plant pathogen Venturia oleaginea. Disease symptoms develop on the upper side of leaves; infected leaves eventually abscise; and in severe epidemics, the trees are completely defoliated. Despite the vast knowledge gained about the pathogen and the disease since it was first described in 1845, observations made in recent years in commercial olive groves in Israel remain unexplained. The long-term objective of this study was to establish guidelines for disease management strategies in commercial olive groves in Israel. To achieve this, we first needed to comprehend the development and progression of the disease in the region. We determined that in each growing season, infections could occur in both autumn and spring. Furthermore, there were two episodes of disease development: the first between the end of autumn and the beginning of winter, and the second between the spring and early summer. The data were utilized to propose a model for peacock eye development which implies that V. oleaginea maintains monocyclic and polyetic characteristics in the Mediterranean climatic conditions prevailing in Israel: the disease is monocyclic because it completes only one disease cycle within a certain growing season; polyetic because infections occurring in one growing season remain asymptomatic until the succeeding season.

This research focused on studying the dynamics of the bacterial pathogen Xylella fastidiosa in almond trees across different developmental stages. The objective was to understand the seasonal distribution and concentration of X. fastidiosa within almond trees. Different tree organs, including leaves, shoots, branches, fruits, flowers, and roots, from ten X. fastidiosa-infected almond trees were sampled over two years. The incidence and concentration of X. fastidiosa were determined using qPCR and isolation. Throughout the study, X. fastidiosa was consistently absent from fruits, flowers, and roots, while it was detected in leaves as well as in shoots and branches. We demonstrate that the absence of X. fastidiosa in the roots is likely linked to the inability of this isolate to infect the peach-almond hybrid rootstock, GF677. X. fastidiosa incidence in shoots and branches remained consistent throughout the year, while in leaf petioles it varied across developmental stages, with lower detection during early and late stages of the season. Similarly, viable X. fastidiosa cells were isolated from shoots and branches at all developmental stages, but no successful isolations were achieved from leaf petioles during the vegetative and nut growth stage. Studying the progression of almond leaf scorch symptoms in trees with initial infections showed that once symptoms emerged on one branch, symptomless branches were likely already infected by the bacterium. Therefore, selectively pruning symptomatic branches is unlikely to cure the tree. This study enhances our understanding of X. fastidiosa dynamics in almond trees and may have practical applications for its detection and control.

This research focused on studying the dynamics of the bacterial pathogen Xylella fastidiosa in almond trees at different developmental stages and in various tree parts. The objective was to understand the annual distribution and concentration of X. fastidiosa within almond trees. Different tree parts, including leaf petioles, annual and perennial shoots, fruit parts, flowers, and roots, from ten X. fastidiosa-infected almond trees were sampled over two years. The distribution and concentration of X. fastidiosa were determined using qPCR and serial dilution plating. Throughout the study, X. fastidiosa was never found in the fruit, flowers, and roots of almond trees, but it was present in leaves and annual and perennial shoots. We show that the inability of X. fastidiosa to colonize roots is likely due to incompatibility with the GF677 rootstock. The presence of X. fastidiosa in shoots remained consistent throughout the year, while in leaf petioles it varied across developmental stages, with lower detection during early and late stages of the season. Similarly, viable X. fastidiosa cells could be isolated from shoots at all developmental stages, while in leaf petioles no successful isolations were achieved during the vegetative and nut growth stage. Examining the development of almond leaf scorch symptoms over time in trees with preliminary infections revealed that once symptoms have appeared on a single branch, other asymptomatic limbs were likely already colonized by the bacterium, hence, selective pruning of symptomatic branches is unlikely to cure the tree. Overall, this study enhances our understanding of X. fastidiosa dynamics in almonds and may have practical applications for its detection and control in almond orchards.

The growth and development of mature, fruit bearing, avocado trees in Menashe Heights region of Israel was retarded from the late-2000s. As time passed the trees decayed gradually, leaves dropped off and eventually the trees died. This process usually spanned 2 to 4 years. The objectives of the study were to: (i) map the spatial distribution of symptomatic avocado trees, (ii) evaluate the potential role of topography in contributing to tree mortality, and (iii) estimate the extent of the tree mortality phenomena in the orchards. On April 2020 we mapped eight sub-units of avocado orchards. In each sub-unit we recorded the state of each tree. We estimated the topographic elevation of each tree using a digital elevation model (DEM). The relationship between the topographic elevation of the trees and the incidence of damaged trees were calculated using a logistic regression. Analysis of the spatial distribution of declining and dying trees reveled that they were not randomly distributed in the orchard and were more abundant in the lower zones of the orchards. Close observations of these zones revealed that the soil in these areas was soaked with water during the winter. Accordingly, we hypothesize that due to limited oxygen supply, root growth and water uptake were impaired, resulting in a negative feedback loop that increases soil water content, reduces aeration, and impairs roots’ ability to absorb oxygen. Such conditions could make the roots more susceptible to soil-borne fungi that had damaged the roots of trees, causing them to deteriorate and eventually die.

Diseases caused by the insect-transmitted bacterium Xylella fastidiosa have been reported in the Americas since the 19th century, causing diseases such as Pierce’s disease of grapevine, almond leaf scorch (ALS), and citrus variegated chlorosis. In the last decade X. fastidiosa was reported from different parts of the world, most notably from southern Italy infecting olives. In 2017, X. fastidiosa was reported to be associated with ALS symptoms in Israel. Here, we investigated the causal agent of ALS in Israel, its genetic diversity and host range, and characterized the temporal and spatial distribution of the disease. X. fastidiosa subsp. fastidiosa sequence type 1 was isolated from symptomatic almond trees and was used to infect almond and grapevine by mechanical inoculation. The pathogen however did not infect olive, peach, cherry, plum, nectarine, clementine, and grapefruit plants. Genomic analysis of local isolates revealed that the local population is derived from a single introduction, and that they are closely related to X. fastidiosa strains from grapevines in California. Distribution analyses revealed that ALS did not expand from 2017 to 2019; however, since 2020, newly symptomatic trees appeared in the tested orchards. Symptomatic trees were located primarily in clusters, and symptoms tended to spread within rows. Our study confirms that X. fastidiosa is the causal agent of ALS in Israel and describes its genetic and host range characteristics. While there is no clear evidence yet for the identity of the vectors in Israel, ALS spread continues to threat the almond and grapevine industries.

Black mold, caused by Aspergillus niger, is the main fungal disease affecting date fruits. Symptoms develop inside the fruit, at the inner space between the pulp and the seed. This study focuses on the etiology of black mold disease in 'Medjoul' fruit. We followed symptoms appearance naturally and after artificial A. niger inoculation at different development and maturity stages. Symptoms developed only in a short period during early fruit ripening. However, fruits are commonly colonized at earlier stages of development. Artificial inoculation of flowers and setting fruitlets increased the level of fruit colonization, while fungicide spraying at these stages decreased colonization. Several weeks following flower inoculation by A. niger, mycelium could be detected on degenerating stigmas and carpels as well as between the fruit and the calyx. Following inoculation with A. niger strain, expressing the Green Fluorescent Protein (A. nigerGFP), the pathogen was detected on stigmas of setting fruitlets but not within the stigmatic tissue or the transmitting tract of the carpel. The A. nigerGFP strain was detected during fruit development below the base of the large fruitlets and above the calyx surrounding the vascular bundle leading into the fruit. The results suggest that A. niger can infect and colonize flowers and setting fruitlets, grows on the degenerating carpels and remains latent at a protected site at the base of the fruit until ripening. Then it induces the typical black mold symptoms. The significance of these results for developing means to cope with the disease is discussed.

Core rot in ‘Red Delicious’ apples, induced by Alternaria alternata, is responsible for significant economic losses in Israel, among other regions. It was previously reported that the pathogen colonizes the loculus (the ovary or seed cavity) of most fruits in Israel. Our working hypothesis was that invasion of A. alternata mycelium colonizing the loculus into the mesoderm (flesh) of apple fruit is governed by the calcium (Ca) concentration in the loculus wall (endocarp): low Ca concentration facilitates pathogen invasion and results in the development of core rot and high Ca concentration precludes the invasion, containing the pathogen in the loculus. The specific objectives of this study were to (i) quantify the relationship between Ca concentration in the endocarp wall and core rot incidence; and (ii) evaluate the effect of means aimed at increasing Ca concentration in the endocarp wall as a tool for core rot suppression. Eight experiments were carried out between 2012 and 2015 in commercial apple groves in northern Israel. The results suggested that high core rot incidence is associated with low Ca concentration in the endocarp wall and vice versa, supporting our working hypothesis. However, our attempts to increase Ca concentration in the endocarp wall by diverting Ca transport from the leaves to the developing fruit and by applying commercial products containing Ca directly to the developing fruit were unsuccessful. Thus, the validity of this approach for suppression of core rot incidence could not be verified.

Soil solarization (SH) is a non-chemical method in which solar heating is used to manage a wide range of soilborne pests. We analyzed several independent studies to assess the efficacy of SH in suppressing a wide range of soilborne pathogens in different agrosystems and under various climatic conditions, and to quantify the added value of combining SH with chemical or non-chemical measures. We analyzed 69 documented experiments and calculated the level of pest management efficacy by SH alone or in combination with either fumigants, organic amendments or biological agents. The analyses were clustered into three groups of soilborne pathogens: (i) various species and formae speciales of Fusarium; (ii) root-knot nematodes; (iii) a group consisting of the pathogens Sclerotium cepivorum, Verticillium, Pyrenochaeta, Rhizoctonia and Pythium. Combining SH with additional measures improved management efficacy, reduced the variance between experimental results, increased the percentage of cases with high management efficacy and reduced the percentage of cases with low management efficacy, compared to SH alone. The efficacy of SH combined with additional measures was not significantly affected by the initial disease pressure. Yield increase in the Fusarium group was positively correlated with disease control efficacy, and the combined measures produced the upper values. These results demonstrate the benefit of combining SH with other control measures in managing soilborne pathogens.

Heart rot, caused by Alternaria alternata, is a major pomegranate disease that impacts production worldwide; most fruits in orchards are colonized by A. alternata but, nevertheless, symptoms are apparent on only a small proportion of the colonized fruits. During the years of our previous research it was noticed that within individual orchards, the incidence of pomegranate fruits exhibiting heart rot symptoms was related to the visual appearance of the trees: trees that appeared visually frail bore more diseased fruits than robust trees. Furthermore, it was noticed that the disease responses of different pomegranate accessions and possibly of different variants of the same cultivar varied markedly. The specific objectives of the present study were: (i) to characterize the relationship between the visual appearance of pomegranate plants or individual stems and the incidence of heart rot and their vulnerability to heart rot; and (ii) to elucidate factors affecting the response of pomegranate fruit to A. alternata. Analysis of heart rot incidence in four orchards in 2014 revealed large differences in heart rot incidence among trees growing side by side in the same orchard; these differences were related to the visual appearance of the pomegranate trees. There were significant differences among germination rates of A. alternata spores in juice prepared from asymptomatic fruits originating from these trees, and comparable differences were found among the acidity levels (pH) of the juices. These differences may reflect differences among the physiological responses of pomegranate trees to heart rot. Fruits collected from the pomegranate collection located in Newe Ya'ar, which comprised 95 accessions in 2015 and 110 accessions in 2016, were also examined. There were differences among the acidity levels (pH) of the juices produced from these fruits and among the germination rates of A. alternata spores in the juices. These differences may reflect variances among the genetic responses of pomegranate accessions to heart rot. Results of studying the relationship between the acidity levels of pomegranate juice and the germination rates of A. alternata spores supported the hypothesis that, apart from pH, a compound(s) present in the juice regulates the germination of A. alternata spores in the juice.