Potassium Phosphonate Induces Resistance in Sweet Chestnut against Ink Disease Caused by Phytophthora Species.

Ink disease, caused by Phytophthora spp., represents a serious threat to sweet chestnuts throughout their distribution area. Among the control strategies, new perspectives have been offered by using potassium phosphonate, which indirectly controls Phytophthora diseases by acting on both host physiology and host-pathogen interactions. In this study, we tested in planta the effectiveness of trunk injection with K-phosphonate against seven different Phytophthora species associated with ink disease. For the two most aggressive species, P. cinnamomi and P. ×cambivora, the treatments were repeated at two different environmental conditions (a mean temperature of 14.5 °C vs. 25 °C) and tree phenology stages. The results obtained in this study demonstrated that K-phosphonate could contain the development of Phytophthora infection in phloem tissues. However, its effectiveness varied based on the concentration applied and the Phytophthora species tested. A concentration of 280 g/L of K-phosphonate was the most effective, and in some cases, callus formation around the necrotic lesion was detected. Overall, this study broadens the knowledge of endotherapic treatments with K-phosphonate as an effective measure for managing chestnut ink disease. Interestingly, the increase in mean temperature had a positive impact on the development of P. cinnamomi lesions on chestnut phloem tissues.

First Report of Dieback of Quercus suber Trees Associated with Phytophthora quercina in Morocco.

Cork oak (Quercus suber L.) is an evergreen tree native to SW Europe and NW Africa. It covers 2·106 ha in the western Mediterranean basin, forms heterogeneous forest ecosystems and represents an important source of income derived from cork production. While in Iberia, Italy, Tunisia and Algeria, drought and several endemic pathogens have been associated with cork oak decline (Moricca et al. 2016; Smahi et al. 2017), in Morocco there is no evidence, apart from overgrazing and human intervention (Fennane and Rejdali 2015), of a pathogen associated with oak decline. In December 2019, extensive dieback and mortality of 60-year-old cork oak trees were observed in a natural stand of ca 150 ha located 5 km east from Touazithe, in Maâmora forest, Morocco (34°13'38''N, 6°14'51''W - 87 m a.s.l.). Two years before, Q. suber seedlings from a local nursery were planted to increase tree density. Symptoms in trees and planted seedlings included chlorosis, reddish-brown discoloration of the whole crown and dieback starting in the upper crown. Root rot and lack of fine roots were observed. Tree mortality was estimated at ca 30%, and disease incidences of trees and seedlings were 45 and 70%, respectively. A Phytophthora species was consistently isolated from the rhizosphere of 3 symptomatic trees randomly selected at the site using leaves as bait (Jung et al. 1996). On carrot agar Phytophthora colonies were uniform and cottonwool-like. Sporangia were typically terminal, with ovoid, and obpyriform shape, mostly papillate, measuring 30.7 ± 4.7 µm length and 22.7 ± 4.1 µm wide. Oogonia were produced in single culture, and they were globose to subglobose, elongated to ellipsoid, 32.1 ± 2.9 µm in diameter and 46.1 ± 4.8 µm in length. Oospores were usually spherical, thick-walled, and measured 28.1 ± 2.4 µm. Antheridia were paragynous, mostly spherical, measuring 12.2 ± 1.4 µm. Isolates had minimum and maximum temperatures of 5 °C and 30 °C, respectively, and a growth optimum at 20 °C. Apart from the small size of sporangia, features were typical of Phytophthora quercina Jung. The identity of a representative strain (TJ1500) was corroborated by sequencing the ITS and mitochondrial cox1 gene regions, and BLAST search in GenBank showed 100% homology with sequences of the ex-type culture of P. quercina (KF358229 and KF358241 accessions, respectively). Both sequences of the representative isolate were submitted to GenBank (accessions OP086243 and OP290549). The strain TJ1500 is currently stored within the culture collections of the Mendel University in Brno and the University of Sassari. Its pathogenicity was verified and compared with a P. cinnamomi strain in a soil infestation test with one-year-old cork oak seedlings (Corcobado et al. 2017). Five months after inoculation, the symptoms described were observed in the seedlings, and fine root weight of plants inoculated with the TJ1500 strain and P. cinnamomi was reduced by 19 and 42%, respectively, in relation to non-inoculated controls. The pathogen was re-isolated from the necrotic roots, thus fulfilling Koch's postulates. So far, P. quercina has been reported associated with chronic mortality of cork oak in new plantations in Spain (Martín-García et al. 2015; Jung et al. 2016) and natural forests in Italy (Seddaiu et al. 2020). To our knowledge this is the first report of P. quercina in Morocco. Givenat Morocco is an important cork producing country, our finding warns about the risk this pathogen poses to Q. suber and other North African oaks.

Phytophthora heterospora sp. nov., a New Pseudoconidia-Producing Sister Species of P. palmivora.

Since 1999, an unusual Phytophthora species has repeatedly been found associated with stem lesions and root and collar rot on young olive trees in Southern Italy. In all cases, this species was obtained from recently established commercial plantations or from nursery plants. Morphologically, the Phytophthora isolates were characterized by the abundant production of caducous non-papillate conidia-like sporangia (pseudoconidia) and caducous papillate sporangia with a short pedicel, resembling P. palmivora var. heterocystica. Additional isolates with similar features were obtained from nursery plants of Ziziphus spina-christi in Iran, Juniperus oxycedrus and Capparis spinosa in Italy, and mature trees in commercial farms of Durio zibethinus in Vietnam. In this study, morphology, breeding system and growth characteristics of these Phytophthora isolates with peculiar features were examined, and combined mitochondrial and nuclear multigene phylogenetic analyses were performed. The proportion between pseudoconidia and sporangia varied amongst isolates and depended on the availability of free water. Oogonia with amphigynous antheridia and aplerotic oospores were produced in dual cultures with an A2 mating type strain of P. palmivora, indicating all isolates were A1 mating type. Phylogenetically, these isolates grouped in a distinct well-supported clade sister to P. palmivora; thus, they constitute a separate taxon. The new species, described here as Phytophthora heterospora sp. nov., proved to be highly pathogenic to both olive and durian plants in stem inoculation tests.