Histopathology of infection and colonization of Quercus ilex fine roots by Phytophthora cinnamomi

Quercus ilex is one of the European forest species most susceptible to root rot caused by the oomycete Phytophthora cinnamomi. This disease contributes to holm oak decline, a particularly serious problem in the ‘dehesas’ ecosystem of the southwestern Iberian Peninsula. This work describes the host–pathogen interaction of Q. ilex and P. cinnamomi, using new infection indices at the tissue level. Fine roots of 6‐month‐old saplings inoculated with P. cinnamomi were examined by light microscopy and a random pool of images was analysed in order to calculate different indices based on the measured area of pathogen structures. In the early stages of invasion, P. cinnamomi colonizes the apoplast and penetrates cortical cells with somatic structures. On reaching the parenchymatous tissues of the central cylinder, the pathogen develops different reproductive and survival structures inside the cells and then expands through the vascular system of the root. Some host responses were identified, such as cell wall thickening, accumulation of phenolic compounds in the middle lamella of sclerenchyma tissues, and mucilage secretion blocking vascular cells. New insights into the behaviour of P. cinnamomi inside fine roots are described. Host responses fail due to rapid expansion of the pathogen and a change in its behaviour from biotrophic to necrotrophic.     

Current situation and characterization of Pseudomonas syringae pv. actinidiae on kiwifruit in Galicia (northwest Spain)

Bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae (Psa), is a disease that is spreading rapidly in several kiwifruit‐producing countries, causing significant economic losses. In 2011, it was detected for the first time in Spain, in the south of Galicia (northwest Spain). Kiwifruit orchards were therefore inspected and sampled in 2011 and 2012 to determine the pathogen distribution, and the isolates obtained were characterized by morphology, fatty acids profile, biochemical tests and molecular techniques. Isolates were obtained from Actinidia deliciosa ‘Hayward’ (from leaves, canes, flower buds, fruits and roots), from A. deliciosa ‘Summer’, from Actinidia chinensis ‘Jin Tao’ (from canes and leaves) and from A. chinensis pollinator ‘Belén’ (from canes). Results of the analysis of the cfl gene (phytotoxin production‐related), the tox–argK gene cluster and phylogenetic analysis of the cts gene demonstrated that all Psa isolates from northwest Spain correspond to the Psa3 population, which includes strains of haplotype 2. This is the first record of Psa3 and haplotype 2 in Spain.     

Effect of salicylic acid treatment on tomato plant physiology and tolerance to potato virus X infection

Salicylic acid (SA) is an inducer of systemic acquired resistance (SAR) and could be a potential candidate in the control of plant virus diseases. In this study we assayed under controlled conditions the potential effect of three doses of exogenous SA treatment on tomato plants infected with Potato virus X (PVX) and measured their effects on: different physiological parameters (gas exchange, stable isotopes, chlorophyll content), the activation of secondary metabolism, viral accumulation and induction of the expression of pathogenesis-related proteins (PRs) such as ß-1, 3-glucanase (PR2) and chitinase (PR3). SA treatment increased the expression of PR2, the activity of phenylalanine ammonia lyase (PAL) and the concentration of antioxidant compounds at 7 days post-treatment. Earlier expression of PR3 compared to PR2 was observed. SA treatment delayed the detection of PVX by ELISA in uninoculated leaves of mechanically infected tomato plants. Although the effect of PVX infection on physiological parameters was weak, moderate SA treatments showed enhanced photosynthesis, particularly for infected plants. The results obtained confirm that SA promotes major changes in the induction of resistance in tomato plants and suggest that treatment with exogenous SA could be considered to reduce the infections caused by PVX.     

Induction of resistance to Sclerotium rolfsii in different varieties of onion by inoculation with Trichoderma asperellum

We evaluated the ability of Trichoderma asperellum Samuels, Lieckfeldt & Nirenberg to induce resistance to the fungal plant pathogen, Sclerotium rolfsii Saccardo, in three onion (Allium cepa L.) varieties. Both the severity of disease and the activities of glucanase, chitinase and peroxidase (enzymes involved in plant resistance) were evaluated in onions inoculated with T. asperellum alone, S. rolfsii alone, or both T. asperellum and S. rolfsii (dual-inoculation) and compared to uninoculated (control) plants. In dual inoculations, the presence of T. asperellum reduced the severity of disease symptoms caused by S. rolfsii. Inoculation with T. asperellum alone increased glucanase, chitinase and peroxidase activity in bulbs, roots and leaves of all three onion varieties compared to uninoculated controls; bulbs of the variety Red Satan (RS) had the highest enzyme activity. In plants inoculated with S. rolfsii alone, enzyme activity was only increased in bulbs and roots compared to uninoculated controls. The highest levels of enzyme activity also occurred only in bulbs and roots of plants that had been dual-inoculated with T. asperellum and S. rolfsii. Plants of the RS variety showed the highest enzyme activities (both constitutive and induced) and showed the lowest severity of disease. Therefore, application of T. asperellum has potential as a biological control alternative to synthetic fungicides for protection of onion crops against infection by S. rolfsii. This protection depends on both constitutive and induced defence responses and varies amongst onion varieties.     

New evidence of intra-race diversity in Fusarium oxysporum f. sp. dianthi populations based on Vegetative Compatibility Groups

Fusarium oxysporum f. sp. dianthi (Fod) causes vascular wilt, the most important carnation disease worldwide. We have analyzed vegetative compatibility in a collection of Fod isolates, obtained from both soils and carnation plants in the most important growing areas in Spain, by pairing all isolates in all possible combinations. Results showed that isolates of race 1 and race 2 were distributed among three Vegetative Compatibility Groups (VCG) which correlated with the molecular Groups previously described. Isolates of race 1 and race 2 in molecular Group I grouped in VCG 0021, isolates of race 1 type were in VCG 0022, and isolates of race 1 and race 2 in molecular Group II constituted a new VCG (002-), not previously reported. Isolates in each VCG contained the same mating type gene (MAT1-1 or MAT1-2), with the exception of the new VCG 002- that contained both idiomorphs. This work identifies a new VCG in Fod populations and reports for the first time the presence of isolates of race 1 and race 2 in the same VCG.     

Seven years of negative detection results confirm that Xylella fastidiosa,the causal agent of CVC,is not transmitted from seeds to seedlings

Knowledge about the mechanism of transmission of systemic pathogens of citrus species is highly important for the safe movement of citrus germplasm, management of citrus mother trees, and also production of young plants. Among systemic pathogens of citrus, Xylella fastidiosa the causal agent of the citrus variegated chlorosis (CVC), is one the most important pathogens causing decline in tree vigour and productivity. Seven-year experiments were conducted to evaluate the hypothesis of seed-to-seedling transmission of X. fastidiosa. This bacterium was found colonizing the fruit (exocarp, central axis and mesocarp) and the seed parts (seed coat and endosperm plus embryo). After 7 years of PCR assay, no positive PCR detection of X. fastidiosa was confirmed in seedlings propagated from the seeds infected with X. fastidiosa. This result demonstrates the lack of seed-to-seedling transmission of this bacterium.     

Isolation and selection of Hemileia Vastatrix antagonists

Organic coffee growing is rapidly increasing in Brazil, and many diseases, especially coffee leaf rust (Hemileia vastatrix), are threatening its production. This study is a first step towards a biocontrol program for coffee rust on organically grown plants. In three organic coffee farms in the state of Minas Gerais, 393 microbial strains including 154 bacterial and 239 fungal strains were isolated from leaves, leaf debris, and soil samples, and in 6 month-old coffee plants, 17 of these isolates reduced both the infection frequency (IF) and the number of H. vastatrix urediniospores produced per leaf (UPL) by more than 70 %. The isolates were identified as eight bacteria isolates, seven Bacillus spp. and one Pseudomonas sp., and nine fungal isolates, four Fusarium spp., two Penicillium spp., one Aspergillus sp., one Acremonium sp. and one Cladosporium sp. Each fungal and bacterial isolate was applied 0, 4, 8, 12 or 16 days before and 0, 4, 8, 12 or 16 days after H. vastatrix inoculation, and the efficiency in reducing both IF and UPL was evaluated. The efficiency was higher and lasted longer when the bacterial isolates were applied before H. vastatrix inoculation. Six Bacillus (B10, B25, B143, B157, B171, B175), two Fusarium (F205, F281), and one Pseudomonas (B286) isolates are potentially efficient as biocontrol agents of H. vastatrix and will be tested using field experiments.