Molecular analyses indicate that both native and exotic pathogen populations serve as sources of novel outbreaks of Cypress Canker Disease

A critical review on bioaerosols-dispersal of crop pathogenic microorganisms and their impact on crop yield

Bioaerosols are potential sources of pathogenic microorganisms that can cause devastating outbreaks of global crop diseases. Various microorganisms, insects and viroids are known to cause severe crop diseases impeding global agro-economy. Such losses threaten global food security, as it is estimated that almost 821 million people are underfed due to global crisis in food production. It is estimated that global population would reach 10 billion by 2050. Hence, it is imperative to substantially increase global food production to about 60% more than the existing levels. To meet the increasing demand, it is essential to control crop diseases and increase yield. Better understanding of the dispersive nature of bioaerosols, seasonal variations, regional diversity and load would enable in formulating improved strategies to control disease severity, onset and spread. Further, insights on regional and global bioaerosol composition and dissemination would help in predicting and preventing endemic and epidemic outbreaks of crop diseases. Advanced knowledge of the factors influencing disease onset and progress, mechanism of pathogen attachment and penetration, dispersal of pathogens, life cycle and the mode of infection, aid the development and implementation of species-specific and region-specific preventive strategies to control crop diseases. Intriguingly, development of R gene-mediated resistant varieties has shown promising results in controlling crop diseases. Forthcoming studies on the development of an appropriately stacked R gene with a wide range of resistance to crop diseases would enable proper management and yield. The article reviews various aspects of pathogenic bioaerosols, pathogen invasion and infestation, crop diseases and yield.

Ultrastructure of Terpene and Polyphenol Synthesis in the Bark of Cupressus sempervirens After Seiridium cardinale Infection

Cypress Canker Disease (CCD) pandemic caused by Seiridium cardinale is the major constraint of many Cupressaceae worldwide. One of the main symptoms of the disease is the flow of resin from the cankered barks. While inducible phloem axial resin duct-like structures (PARDs) have recently been characterized from an anatomical point of view, their actual resin production is still being debated and has never been demonstrated. Although the involvement of polyphenolic parenchyma cells (PP cells) in the bark of Cupressus sempervirens after S. cardinale infection was revealed in one of our previous studies using light microscopy, their evolution from the phloem parenchyma cells is yet to be clarified. This study investigated functional and ultrastructural aspects of both PARD-like structures and PP cells by means of more in-depth light (LM) and fluorescence microscopy (FM) combined with histochemical staining (using Sudan red, Fluorol Yellow, NADI Aniline blue black, and Toluidine blue staining), in addition to Transmission Electron Microscope (TEM). Two-year-old stem sections of a C. sempervirens canker-resistant clone (var. “Bolgheri”), artificially inoculated with S. cardinale, were sampled 5, 7, 14, 21, and 45 days after inoculation, for time-course observations. FM observation using Fluorol yellow dye clearly showed the presence of lipid material in PARD-like structures lining cells of the cavity and during their secretion into the duct space/cavity. The same tissues were also positive for NADI staining, revealing the presence of terpenoids. The cytoplasm of the ducts' lining cells was also positive for Sudan red. TEM observation highlighted the involvement of plastids and endoplasmic reticulum in the production of terpenoids and the consequent secretion of terpenoids directly through the plasma membrane, without exhibiting vesicle formation. The presence of a high number of mitochondria around the area of terpenoid production suggests that this process is active and consumes ATP. The LM observations showed that PP cells originated from the phloem parenchyma cells (and possibly albuminous cells) through the accumulation of phenolic substances in the vacuole. Here, plastids were again involved in their production. Thus, the findings of this work suggest that the PARD-like structures can actually be considered PARDs or even bark traumatic resin ducts (BTRD).

Genetic Variation for Economically Important Traits in Cupressus lusitanica in New Zealand

Increasing productivity and tolerance against cypress canker disease is an important goal in the Mexican white cypress breeding program in New Zealand, and screening has been in place since 1983. Cypress canker disease is caused by Seiridium cardinale and Seiridium cupressi, the current study presents the results of two progeny trials within the breeding program in the North Island of New Zealand. The trials were established as open-pollinated progeny tested and were assessed for diameter at breast height, branch size, canker severity score, malformation score, and stem straightness score and acceptability score. Heritability estimates were moderate ranging from 0.21 to 0.41 for diameter at breast height and from 0.14 to 0.31 for canker severity score. Stem form attributes showed heritability from 0.08 (malformation) to 0.38 (straightness). No trait showed any significant G × E interaction between investigated sites. This was supported by the very strong genetic correlations estimated between the traits recorded in Welcome Bay and Matata trials. Unfavourable genetic correlations ranging from 0.25 to 0.46 were found between diameter at breast height and canker severity score, indicating that the continued selection for genotypes with improved diameter at breast height would also increase susceptibility to cypress canker. Additionally, unfavourable genetic correlations ranging from 0.52 to 0.73 were found between branch size and diameter at breast height and should be considered in selection programs. The moderate heritability estimated for canker severity score indicates that breeding values for this trait could be predicted with acceptable accuracy and included in the breeding program for Cupressus lusitanica, enabling the identification of genotypes with tolerance to canker severity to be deployed to locations where cypress canker is present in New Zealand.

Risk assessment of the entry of Pantoea stewartii subsp. stewartii on maize seed imported by the EU from the USA

Following a request from the European Commission, the EFSA Panel on Plant Health performed a risk assessment of the entry of Pantoea stewartii subsp. stewartii on maize seed imported by the EU from the USA. This pest is a Gram-negative bacterium which causes Stewart's vascular wilt and leaf blight of maize (including sweet corn), a disease responsible for serious crop losses throughout the world. The following scenarios were considered: scenario A0 (current practice), scenario A1 (US request for modification of EU conditions for derogation), and scenario A2 (EU conditions for derogation). Results from the quantitative seed pathway model presented here show that, despite the low rates of plant-to-seed and seed-to-seedling transmission that have been reported in the literature for Stewart's wilt, given the amount of traded seed, and in the case of voluntary (i.e. not mandatory) inspections of seed production fields at the origin (i.e. scenario A0), the frequency of introducing the disease is in the order of magnitude of some hundred introductions per year (median number). The EU conditions for derogation would lead to a decrease in the likelihood of entry compared to scenarios A0 (about 10,000 times fewer introductions) and A1 (about 2,000 times fewer introductions). This protective effect is mainly due to the requirement that only genotypes resistant to Stewart's wilt are traded, with the additional field inspection (two instead of one per season) providing additional reassurance. The Panel also concluded that seed lot inspections, as currently carried out (e.g. with a sample of 400 seeds) are not likely to lead to a relevant reduction in the level of infected imported maize seed, given the low prevalence of Stewart's wilt at the origin. If, however, there is aggregation in infection among consignments, inspection would work towards identifying the highly infected consignments. Recently, outbreaks of Stewart's wilt have occurred in Italy (Emilia Romagna, Friuli, Lombardy and Veneto). A review is provided of the available information to assess the possible role of seed imports in these outbreaks.