Characterization of Zymoseptoria tritici populations in Belarus by morphologic and cultural features

Septoria leaf blotch caused by Zymoseptoria tritici is one of the most harmful diseases in Belarus. Isolates of the pathogen were obtained from northern, central and southern populations: in 2018–2019, whichvaried significantly among in the structure of colonies. The rate of fungal isolates forming yeast-like colonies decreased from 55.0 % in the North of Belarus to 6.7 % in the South, whereas incidence of filamentous isolates increased from 31.3 % to 80.0 %, respectively. In the northern population, phenotypic diversity was high, while in the southern population it was the lowest (Shannon’s index was 1.53 and 1.14, respectively).

Identification and pathogenicity of Alternaria species associated with leaf blotch disease and premature defoliation in French apple orchards

Leaf blotch caused by Alternaria spp. is a common disease in apple-producing regions. The disease is usually associated with one phylogenetic species and one species complex, Alternaria alternata and the Alternaria arborescens species complex (A. arborescens SC), respectively. Both taxa may include the Alternaria apple pathotype, a quarantine or regulated pathogen in several countries. The apple pathotype is characterized by the production of a host-selective toxin (HST) which is involved in pathogenicity towards the apple. A cluster of genes located on conditionally dispensable chromosomes (CDCs) is involved in the production of this HST (namely AMT in the case of the apple pathotype). Since 2016, leaf blotch and premature tree defoliation attributed to Alternaria spp. have been observed in apple-producing regions of central and south-eastern France. Our study aimed to identify the Alternaria species involved in apple tree defoliation and assess the presence of the apple pathotype in French orchards. From 2016 to 2018, 166 isolates were collected and identified by multi-locus sequence typing (MLST). This analysis revealed that all these French isolates belonged to either the A. arborescens SC or A. alternata. Specific PCR detection targeting three genes located on the CDC did not indicate the presence of the apple pathotype in France. Pathogenicity was assessed under laboratory conditions on detached leaves of Golden Delicious and Gala apple cultivars for a representative subset of 28 Alternaria isolates. All the tested isolates were pathogenic on detached leaves of cultivars Golden Delicious and Gala, but no differences were observed between the pathogenicity levels of A. arborescens SC and A. alternata. However, the results of our pathogenicity test suggest that cultivar Golden Delicious is more susceptible than Gala to Alternaria leaf blotch. Implications in the detection of the Alternaria apple pathotype and the taxonomic assignment of Alternaria isolates involved in Alternaria leaf blotch are discussed.

Autophagy Inhibits Intercellular Transport of Citrus Leaf Blotch Virus by Targeting Viral Movement Protein

Autophagy is an evolutionarily conserved cellular-degradation mechanism implicated in antiviral defense in plants. Studies have shown that autophagy suppresses virus accumulation in cells; however, it has not been reported to specifically inhibit viral spread in plants. This study demonstrated that infection with citrus leaf blotch virus (CLBV; genus Citrivirus, family Betaflexiviridae) activated autophagy in Nicotiana benthamiana plants as indicated by the increase of autophagosome formation. Impairment of autophagy through silencing of N. benthamiana autophagy-related gene 5 (NbATG5) and NbATG7 enhanced cell-to-cell and systemic movement of CLBV; however, it did not affect CLBV accumulation when the systemic infection had been fully established. Treatment using an autophagy inhibitor or silencing of NbATG5 and NbATG7 revealed that transiently expressed movement protein (MP), but not coat protein, of CLBV was targeted by selective autophagy for degradation. Moreover, we identified that CLBV MP directly interacted with NbATG8C1 and NbATG8i, the isoforms of autophagy-related protein 8 (ATG8), which are key factors that usually bind cargo receptors for selective autophagy. Our results present a novel example in which autophagy specifically targets a viral MP to limit the intercellular spread of the virus in plants.

Evaluation of Soybean Entries in the Pan-African Trials for Response to Coniothyrium glycines, the Cause of Red Leaf Blotch

Red leaf blotch, caused by the fungus Coniothyrium glycines, is an important disease of soybean known to cause yield losses across soybean growing regions in Africa. Fungicides are one option to manage this disease, but utilization of host resistance may be a better option suited for smallholder soybean farmers in Africa. Fifty-nine soybean entries were evaluated for red leaf blotch severity in nine field locations in Ethiopia, Kenya, Uganda, and Zambia. Disease incidence was 100% and disease severity differed (P < 0.01) among entries at eight of the nine locations. Mean severity rating ranged from 1.4 to 3.2 based on 0 to 5 scale with higher disease severities recorded in Ethiopia followed by Zambia. Seven of the 59 entries were common to all nine locations and had severity ranging from 1.7 to 2.9. The cultivar SC Signal had the lowest red leaf blotch severity ratings in the combined analysis. Based on correlations of weather variables to red leaf blotch severity, mean rainfall from planting to assessment date had a positive correlation (r = 0.90; P = 0.010) as did wind speed (r = 0.74; P = 0.0235). Other variables, such as temperature and relative humidity, did not correlate to red leaf blotch severity. This is the most comprehensive report to date on the disease incidence in the region, which for the first time demonstrates an association between rainfall and wind speed with red leaf blotch severity. It also represents the first extensive report evaluating soybean genotypes for resistance against red leaf blotch under multiple environments.

First Report of Diaporthe cercidis Causing Leaf Blotch of Acer pictum subsp. mono in China

Acer pictum subsp. mono (Maxim.) H. Ohashi is a common deciduous tree species that is widely distributed in Northeast and Northern China, including all provinces of the Yangtze River Basin (Liu et al. 2014). A foliar disease, with an incidence of ~90% (19/21 trees), occurred on A. pictum subsp. mono in a community park, Nanjing, Jiangsu, China in July 2019. On average, ~80% of the leaves per individual tree were infected by this disease. The symptoms initially appeared as brown, necrotic lesions at leaf tips, and half the leaf would become dark brown with time, and finally almost all of leaves were infected. Small pieces of leaf tissue (3 to 5 mm2) cut from the lesion margins were surface-sterilized in 75% ethanol for 30 s and 1% NaClO for 90 s, rinsed with sterile H2O three times, and placed on potato dextrose agar (PDA) at 25°C in the dark. The same fungus was isolated from 92% of the samples. The pure cultures were obtained by single-spore isolation. Three representative isolates (WJF1, WJF3 and WJF4) were obtained, and WJF1 was deposited in China’s Forestry Culture Collection Center (CFCC 54806), and WJF3 and WJF4 were deposited at the Nanjing Forestry University (NFU 083 and NFU 084). The culture on PDA was white, with white vigorous aerial mycelia at the edge. Black pycnidia developed on the alfalfa stems at 25°C under a 14/10 h light/dark cycle for 20 days. Conidiophores were hyaline, branched, septate, straight, 16.4–34.7 × 1.5–3.0 μm (n = 30). Conidiogenous cells were 9.0–24.6 × 1.3–2.3 μm (n = 30). Alpha conidia were 7.0 ± 0.6 × 2.2 ± 0.2 μm (n = 30), fusiform, hyaline, smooth and multi-guttulate. Beta conidia were 25.5 ± 4.3 × 1.3 ± 0.1 μm (n = 30), hyaline, smooth and hamate. Morphological characters of all three isolates matched those of Diaporthe spp. (Gomes et al. 2013). DNA of three isolates were extracted and the internal transcribed spacer region (ITS), partial sequences of elongation factor 1-alpha (EF1-α), calmodulin (CAL), beta-tubulin (β-tub) and histone H3 (HIS) genes were amplified with primers ITS1/ITS4 (White et al. 1990), EF1-728F/EF1-986R and CAL228F/CAL737R (Carbone et al. 1999), βt2a/βt2b and CYLH3F/H3-1b (Glass and Donaldson, 1995, Crous et al. 2004), respectively. The sequences of WJF1, WJF3 and WJF4 were deposited in GenBank (WJF1: Accession Nos. MW301339 for ITS, MW363932 to MW363935 for EF1-α, β-tub, HIS, and CAL; WJF3: MW453062 and MW561566 to MW561569; WJF4: MW453063 and MW561570 to MW561573). BLAST results showed that the ITS, EF1-α, β-tub, HIS, and CAL sequences of WJF1 were similar with sequences of Phomopsis liquidambari C.Q. Chang, Z.D. Jiang & P.K. Chi JQ676191 (identity = 540/540; 100%), D. huangshanensis H. Zhou & C.L. Hou MN224671 (identity = 291/292; 99%), D. pescicola Dissan., J.Y. Yan, Xing H. Li & K.D. Hyde MK691230 (identity = 438/438; 100%), D. spinosa Y.S. Guo & G.P. Wang MK726170 (identity = 437/438; 99%), D. cercidis C.M. Tian & Qin Yang MK691114 (identity = 452/452; 100%), respectively. BLAST results of WJF3 and WJF4 are list in Table 1. A maximum likelihood and Bayesian posterior probability analyses using IQtree v. 1.6.8 and Mr. Bayes v. 3.2.6 with the concatenated sequences placed WJF1, WJF3 and WJF4 in the clade of D. cercidis. Based on the five-locus phylogeny and morphology, WJF1, WJF3 and WJF4 were identified as D. cercidis. The pathogenicity of three isolates were tested on potted 3-yr-old seedlings of A. pictum subsp. mono, grown in a greenhouse. Healthy leaves were wounded with a sterile needle and then inoculated with 10 μL of conidial suspensions (106 conidia/mL). Control leaves were treated with sterilized H2O. A total of twelve seedlings were used for the tests, 3 seedlings per treatment, and five leaves were inoculated per seedling. Each plant was covered with a plastic bag after inoculation and sterilized H2O was sprayed into the bag twice/day to maintain humidity and kept in a greenhouse at the day/night temperatures at 25 ± 2°C/16 ± 2°C. In 5 days, all the inoculated leaves had lesions similar to those observed in the field. D. cercidis was reisolated from the lesions of the inoculated leaves and was confirmed based on morphological characteristics and ITS sequence analysis. No symptoms were observed on the control leaves, and no fungus was isolated from them. D. cercidis was previously reported on twigs of Cercis chinensis (Yang et al. 2018) and causing pear shoot canker (Guo et al. 2020). This is the first report of D. cercidis causing leaf blotch on A. pictum subsp. mono. Identification of the pathogen is imperative for diagnosing and controlling this potentially high risk disease on A. pictum subsp. mono and also for the future studies.