ABSTRACT
Alder decline caused by Phytophthora species is considered one of Europe´s most important diseases of natural ecosystems. In Spain, P. x alni, P. uniformis, P. x multiformis and P. plurivora have been detected in association with root and collar rot in riparian alder populations (Pintos et al. 2010, 2012, 2017; Haque et al. 2014). During the active growth periods 2020-2021, a field survey of the newly designed species Alnus lusitanica (Vít et al. 2017) was carried out at the most symptomatic areas along the Miño-Sil river basin in Galicia and León (NW-Spain). Samples of bark, root, rizosphere soil of declining trees, and river water were collected. Symptoms, similar to those caused by other Phytophthora species, included low leaf density of the canopy, generally with smaller and chlorotic leaves, and branches with dry tips. In some cases, the presence of cankers on the trunk were observed (Figure 1). Necrotic lesions were transferred onto V8-PARPH, a Phytophthora selective medium. Soil and water were baited with carnation petals. Three isolates of one Phytophthora species was recovered: two from the roots of trees, and one from river water at three different sites. The isolates were transferred to V8 agar and incubated a 22ºC in the dark. Colonies had petaloid patterns, and their optimum growth temperature ranged from 25 to 30 °C. In soil extract, non-caducous, non-papillate and ovoid sporangia were produced. Sporangia averaged 41.0×29.4 µm with a length/breadth ratio of 1.39. Internal proliferation occurred (Figure 2). No sexual structures were observed . Genomic DNA was extracted from mycelium obtained from pure cultures of three Phytophthora isolates. The ITS region of the ribosomal DNA template was performed by nested PCR using DC6 (Cooke et al. 2000) and ITS4 (White et al. 1990) in the first round, and ITS6 (Cooke et al. 2000) and ITS4 in the second. The mitochondrial gene cox1 was amplified with primers CoxF4N and CoxR4N (Kroon et al. 2004). BLASTn analyses showed 100% identity to the type of P. lacustris (AF266793, 156 matching bp) for ITS, and 100% identity to the ex-type of P. lacustris (MH136916, 596 matching bp) for the cox1 sequence. Phylogenetic analysis indicated that our isolates were localized in the same evolutionary branch as P. lacustris based on consense sequences of ITS and cox1 sequences. The ITS and cox1 sequences generated in this study were deposited in GenBank with accession number OP588369 and OP548051, and one isolate isolated from root (CECT 21212) was submitted to the Spanish Type Culture Collection (Paterna, Valencia). Pathogenicity tests with isolate CECT 21212 were conducted on ten 3-year-old alders (A. lusitanica) growing in free draining 5 L pots. One shallow cut was made into the cambium at the root collar level. A colonized 5-mm mycelial agar plug, from a 7-day-old culture was inserted into every wound (mycelial surface face-down) and sealed with Parafilm®. Five control plants were inoculated with a sterile agar plug. Plants were kept in a controlled chamber at 25 ºC and 80% humidity. After a 3-week incubation period, inoculated plants showed dieback symptoms and necrosis of the inner bark tissue (Figure 3). Lesion lengths ranged from 2 to 8 cm. Control plants remained symptomless. P. lacustris was recovered from all inoculated plants, but not from the control ones. To our knowledge, this is the first report of P. lacustris causing root rot on alders in Spain. This new detection represents an increased threat to riparian alders by the presence of an additional Phytophthora species associated with alder decline, since P. lacustris can readily adapt to a wide variety of climatic conditions with the ability to infect different hosts (Nechwatal et al. 2013).
ABSTRACT
About 60% of the nut production of sweet chestnut (Castanea sativa Mill.) in Europe originates in Spain (FAOSTAT, 2022), mostly (91%) in Galicia (NW Spain). In September 2021, premature fall of immature chestnut burrs and nuts of C. sativa was observed in eight orchards of Pontevedra and Ourense (provinces of Galicia). Chestnut green burrs had turned brown and fallen off, and the nuts showed brown lesions in kernels and embryos. Some nuts had become mummified. Symptomatic samples of burrs and nuts, including mummified fruits, were collected. Small pieces of samples were surface-disinfected with 2% sodium hypochlorite, and plated onto potato dextrose agar (PDA) plates. Colonies were creamy white to gray or light brown, and presented a woolly to felty mycelium with a dense development in concentric circles. Brownish to black conidiomata, produced abundantly, were globose to sub-globose. Conidia were hyaline, oval, obovoid, fusoid and multi-guttulate, and 6.6±0.78 [5.07 to 9.01] µm x 3.2±0.43 [2.41 to 4.38] µm in size. These features matched those described for Gnomoniopsis smithogilvyi (Shuttleworth et al. 2012), syn. G. castaneae (Visentin et al. 2012; Shuttleworth et al. 2015). Genomic DNA was extracted from mycelium developed in 22 burrs and nuts, and 30 pure cultures. The rDNA internal transcribed spacer (ITS), fragments of the ß-tubulin (TUB2) and the translation elongation factor-1α (TEF-1α) genes were amplified using ITS1F (Gardes and Bruns 1993) and ITS4 (White et al. 1990), T1/Bt2b (O'Donnell and Cigelnik 1997), and EF1-728F/EF1-986R (Carbone and Kohn 1999) primers, respectively. Two isolates (EFA 924A, EFA962.4A) were deposited in the Spanish Type Culture Collection (Paterna, Valencia), and their sequences submitted to GenBank (accession nos.: ITS: OM319846, OM319848; TUB2: OM417078, OM417080; TEF-1α: OM417081, OM417083). BLASTn analyses showed: for ITS and TEF-1α sequences, ≥99.7% identity to the ex-type of G. smithogilvyi (ITS: JQ910642, 474 matching bp; TEF-1α: JQ910645, 335-338 matching bp), and for TUB2 sequences, ≥99.1% identity to G. castaneae (LN999975, 446-453 matching bp). Pathogenicity tests were carried out on surface disinfected nuts of Castanea sativa `Raigona´. A superficial wound was made in the pericarp of each nut. A 2-mm mycelial plug of a 7-days-old culture of G. smithogilvyiwas then inoculated: twenty nuts with isolate EFA924A and twenty with isolate EFA962.4A. Twenty nuts inoculated with sterile agar were used as control. The holes were sealed with Parafilm®. Nuts were incubated in a moist chamber at 22±2°C. Two replicated tests were carried out. Four inoculated and four control nuts were inspected for the presence and progress of rot symptoms every seven days. Three weeks after inoculation, all remaining inoculated nuts were completely rotted, whereas all control nuts remained healthy. Gnomoniopsis smitholgilvyi was reisolated from all inoculated nuts, and it was not recovered from the controls. This pathogen causes chestnut brown rot on sweet chestnut worldwide (EPPO, 2022), causing also shoot cankers on chestnut (Lione et al, 2019). This is the first report of G. smithogilvyi causing chestnut brown rot on nuts and burrs of C. sativa in Spain. Future studies on the incidence of this pathogen and its impact on chestnut yield should be carried out in the main European producing countries, i.e. Spain, Italy, Portugal and Greece, where the disease has been detected and represents an emerging threat to chestnut production.
