Rootstock Effects on Pistachio Trees Grown in Verticillium dahliae-Infested Soil

Contrasting allelic effects for pistachio salinity tolerance in juvenile and mature trees

Breeding perennial tree crops often requires prediction of mature performance from juvenile data. To assess the utility of juvenile screens to predict salinity tolerance of mature pistachio trees, we compared performance of 3-month ungrafted seedlings and 4-year-old grafted rootstocks under salinity stress. The QTL allele associated with higher salt exclusion from seedling leaves conferred lower growth in saline field conditions, suggesting that mapping QTL in seedlings may be easier than discerning the optimal allele for field performance.

Avoidant/resistant rather than tolerant olive rootstocks are more effective in controlling Verticillium wilt

The identification of rootstocks of low susceptibility to Verticillium dahliae can become a valuable procedure to achieve effective control of Verticillium wilt in the olive grove. This not only involves the identification of suitable genotypes, but also the study of the interaction between the rootstock and the grafted scion. Thus, a rootstock that prevents or minimizes V. dahliae proliferation (avoidance/resistance strategy) can have very different effects on a susceptible scion compared to a rootstock that shows few or no symptoms despite being infected (tolerance strategy). Both resistance and tolerance mechanisms have been recently identified in wild olive genotypes with low susceptibility to V. dahliae. When used as rootstocks of the highly susceptible variety 'Picual', we found that resistant genotypes, including the cultivar 'Frantoio', were more effective than tolerant genotypes in controlling Verticillium wilt. Furthermore, tolerant genotypes were as ineffective as susceptible or extremely susceptible genotypes in controlling Verticillium wilt. We also identified rootstock-scion combinations with behaviours that were not expected according to the degree of susceptibility previously observed in the non-grafted rootstock. Although the rootstocks were able to control Verticillium wilt according to its degree of susceptibility to V. dahliae, the ability to control the infection was not adequately transferred to the grafted scion. Our results confirmed that: the degree of susceptibility to Verticillium wilt of an olive variety does not predict its performance as a rootstock; to use a very low susceptible genotype as rootstock of a susceptible scion increases the susceptibility of the genotype used as rootstock; in any case, avoidant/resistant rootstocks are more effective than tolerant rootstocks in reducing the susceptibility of the grafted plant to V. dahliae.

Advances in Rootstock Breeding of Nut Trees: Objectives and Strategies

The production and consumption of nuts are increasing in the world due to strong economic returns and the nutritional value of their products. With the increasing role and importance given to nuts (i.e., walnuts, hazelnut, pistachio, pecan, almond) in a balanced and healthy diet and their benefits to human health, breeding of the nuts species has also been stepped up. Most recent fruit breeding programs have focused on scion genetic improvement. However, the use of locally adapted grafted rootstocks also enhanced the productivity and quality of tree fruit crops. Grafting is an ancient horticultural practice used in nut crops to manipulate scion phenotype and productivity and overcome biotic and abiotic stresses. There are complex rootstock breeding objectives and physiological and molecular aspects of rootstock-scion interactions in nut crops. In this review, we provide an overview of these, considering the mechanisms involved in nutrient and water uptake, regulation of phytohormones, and rootstock influences on the scion molecular processes, including long-distance gene silencing and trans-grafting. Understanding the mechanisms resulting from rootstock × scion × environmental interactions will contribute to developing new rootstocks with resilience in the face of climate change, but also of the multitude of diseases and pests.

Assessment of changes in physiological and biochemical traits in four pistachio rootstocks under drought, salinity and drought + salinity stresses

In this study, 7-month-old UCB-1, Badami, Ghazvini and Kale-Ghouchi pistachio rootstocks were exposed to control, drought, salinity and drought + salinity environments for 60 d. Total chlorophyll and total carotenoid contents decreased in all cultivars under drought, salinity and drought + salinity stresses. Under drought and salinity stresses, alone or in combination, Na⁺ and Cl^- ions increased in all four pistachio rootstocks, while K⁺ ion decreased only in Ghazvini and Kaleh-Ghouchi cultivars. The enzyme activities of ascorbate peroxidase, polyphenol oxidase, catalase and guaiacol peroxidase increased in all cultivars when subjected to all three stresses with the exception of the ascorbate peroxidase activity in Kale-Ghouchi cultivar during drought stress. Oxidative stress parameters including electrolyte leakage, malondialdehyde, other aldehydes and hydrogen peroxide increased under all three stress conditions in all genotypes. The content of proline, total free amino acids and total soluble carbohydrates were enhanced under drought, salinity and drought + salinity stresses, whereas the protein content decreased in all pistachio rootstocks. In all evaluated traits, except for the K⁺ ion content and APX activity, the highest impacts was seen for drought + salinity > salinity > drought stresses, respectively. For the first time, we have proven that K⁺ ion content has a positive correlation with the ascorbate peroxidase, polyphenol oxidase, catalase and guaiacol peroxidase enzymes activities under drought + salinity stress. Finally, based on the bi-plot and cluster analyses, we have selected the UCB-1 > Badami > Ghazvini > Kale-Ghouchi cultivars as the most tolerant pistachio rootstocks under drought + salinity stress, respectively.

Macrophomina Crown and Root Rot of Pistachio in California

In this study, declining pistachio rootstocks were detected in newly planted commercial pistachio orchards in Kern County, California. Symptoms were characterized by wilted foliage combined with crown rot in the rootstock. From diseased trees, 42 isolates were obtained, and all had similar cultural and morphological characteristics of Macrophomina phaseolina. Analyses of nucleotide sequences of three gene fragments, the internal transcribed spacer region (ITS1-5.8S-ITS2), partial sequences of β-tubulin, and translation elongation factor 1-α (TEF1) confirmed this identification, and 20 representative isolates are presented in the phylogenetic study. Testing of Koch's postulates showed that M. phaseolina, when inoculated to stems and roots of the pistachio rootstocks using mycelial plugs or a microsclerotial suspension, is indeed pathogenic to this host. The widely used clonal University of California Berkeley I (UCBI) rootstock appeared highly susceptible to M. phaseolina, suggesting that this pathogen is an emerging threat to the production of pistachio in California. This study confirmed the association of M. phaseolina with the decline of pistachio trees and represents the first description of this fungus as a crown rot-causing agent of pistachio in California.

Rootstock scion somatogenetic interactions in perennial composite plants

The ancient plant production practice of grafting which instantly imparts new physiological properties to the desirable scion still remains shrouded in mystery. Yet, grafting remains a widely used technique in the production of several horticultural species. In a composite grafted plant, rootstocks control many aspects of scion growth and physiology including yield and quality attributes as well as biotic and abiotic stress tolerance. Broadly, physical, physiological, biochemical and molecular mechanisms have been reviewed to develop an integrated understanding of this enigmatic process that challenges existing genetic paradigms. This review summarizes the reported mechanisms underlying some of the economically important traits and identifies several key points to consider when conducting rootstock scion interaction experiments. Study of the somatogenetic interactions between rootstock and scion is a field that is ripe for discovery and vast improvements in the coming decade. Further, utilization of rootstocks based on a better understanding of the somatogenetic interactions is highly relevant in the current agricultural environment where there is a need for sustainable production practices. Rootstocks may offer a non-transgenic approach to rapidly respond to the changing environment and expand agricultural production of annual and perennial crops where grafting is feasible in order to meet the global food, fiber and fuel demands of the future.

Field Resistance to Verticillium Wilt in Selected Olive Cultivars Grown in Two Naturally Infested Soils

The resistance of 11 olive cultivars to Verticillium dahliae was assessed in two experimental field trials. One-year-old rooted olive cuttings from the World Olive Germplasm Bank (IFAPA research center, Córdoba, Spain) were planted in a heavily infested field in Utrera (Sevilla province) and in a moderately infested field in Andújar (Jaén province) of southern Spain. Plants were assessed for Verticillium wilt resistance during 22 months based on disease severity and tree growth. Severe disease symptoms were observed 6 months after planting in both trials. Twenty months after planting in the heavily infested soil, V. dahliae had killed nearly all of the trees of 'Bodoquera', 'Cornicabra', 'Manzanilla de Sevilla', and 'Picual', demonstrating the elevated risk of planting susceptible cultivars in a soil heavily infested with V. dahliae. 'Arbequina', 'Koroneiki', 'Sevillenca', and especially 'Frantoio', 'Empeltre', and 'Changlot Real' showed a high level of disease resistance. However, all of them were affected by the disease. Although the field results confirmed the level of resistance previously obtained for these olive genotypes under controlled conditions, there were some discrepancies. This information will be useful in managing the disease and also in selecting new cultivars for the breeding of Verticillium wilt resistance.

Study on the role of common weeds in survival of Verticillium dahliae the causal agent of cotton wilt disease

This study was carried out to investigate the roles of common weeds in the survival of Verticillium dahliae and the incidence of cotton wilt disease in Moghan and Neishabour area of Iran during 2003-2005 cropping seasons. The design of the experiment was Randomized Complete Blocks (RCB) with ten treatments (No. of weeds) and four replications (No. of cotton fields). Populations of V. dahliae in the roots of weeds and their surrounding soil was determined every year and were compared in different treatments. The cotton wilt disease index was also evaluated in different fields in each experimental site. Results indicated that the fungal population was variable depending on weed species, experimental site and the year of study. In general, Pigweed (Amaranthus retroflexus), Nightshde (Solanum nigrum), Spiny cockleburr (Xanthium spinosum) and common purslane (Portulaca oleraceea) showed the highest fungal populations in their roots and surrounding soil. Disease index in Moghan cotton fields in different years varied and were higher than Neishabour fields.

Identification of Fusarium solani f. sp. cucurbitae Race 1 and Race 2 with PCR and Production of Disease-Free Pumpkin Seeds

Fusarium solani f. sp. cucurbitae causes a fruit rot of cucurbits and is classified into two races that are actually distinct species: F. solani f. sp. cucurbitae race 1 (Fsc1) and race 2 (Fsc2). Because Fsc1 and Fsc2 are not easily distinguished morphologically, we developed a polymerase chain reaction (PCR) assay for rapid identification. Taxon-specific primers were designed from translation elongation factor 1-α sequences. Because clean seed is critical for disease control, we conducted experiments to determine if we could predict whether seed would be infected based on a visual rating of the fruit at harvest. In two trials in commercial pumpkin fields, eight fruit in each of four categories were selected: asymptomatic fruit, mildly infected fruit, severely infected fruit but without lesions extending into the seed cavity, and severely infected fruit with at least one lesion extending into the seed cavity. Isolates from both lesions and seed were identified as Fsc1 based on the PCR assay. No infected seed were recovered from fruit in which the surface was lesion-free or in which a lesion extended less than midway through the fruit flesh. Consequently, a rapid, visual inspection and exclusion of symptomatic fruit should be sufficient to obtain uninfected seed, even in infested fields.