Morphological, pathogenic and genetic diversity in Diplodia seriata associated with black rot canker of apple in India.

Apple cankers are extremely destructive diseases threatening the global apple industry through direct and indirect losses. The population structure of the pathogens is of paramount significance for the development of efficient management strategies. Therefore, phenotypic, pathogenic, and genetic diversity of Diplodia seriata causing black rot canker of apple was investigated in this study. All the isolates were included for investigating the in vitro mycelial growth, conidial dimensions, and pathogenic variability on two-year-old potted apple seedlings. The ISSR approach was used to investigate the molecular diversity of D. seriata. Mycelial growth rates were found to vary significantly amongst the isolates; however, there were no major variations seen between the different geographical groupings of isolates. Pathogenicity tests revealed variations in the size of cankers among the isolates indicating the presence of virulence variability. The isolates were segregated into three virulence groups based on canker length. The Bayesian analyses of ISSR data divided the isolates into two genetic clusters. The genetic clustering of the isolates revealed no relationship with geographical origin of the isolates. Furthermore, no direct relationship of genetic clustering was observed with morphological or pathogenic variability. The ISSR primers revealed very high level of variability in D. seriata; however, no distinct populations of the pathogen existed which is an indication of high level of gene flow between the diverse geographical populations. According to our knowledge, this is the first thorough investigation on the diversity of D. seriata associated with apple black rot canker in India.

First report of powdery mildew caused by Phyllactinia pyri-serotinae Sawada on pear (Pyrus communis L.) from India.

In August 2020 powdery mildew was observed on pear cv. Fertility at the University research field in Shalimar, Srinagar (J&K), India (34° 08' 30.5'' N and 74° 51' 42.0'' E) with a disease incidence up to 30% (100 leaves observed from ten trees). White irregularly shaped fungal colonies were observed on the abaxial leaf surface which latter covered the whole leaf surface and developed black chasmothecia. The affected leaves appeared brittle, slightly curved upwards and dropped prematurely. Mycelium was hypophyllous, septate and measured 2.0 to 5.0 µm in width. Appressoria were nipple shaped, solitary or present in opposite pairs. Conidiophores were erect, up to 440.0 µm long (n=50), mostly centrally on upper surface of mother cells. Conidiophore foot-cells were filiform, followed by 1 to 3 shorter cells, producing single conidia at the tip. Conidia were hyaline, lanceolate, with a non-papillate rounded apex, measuring55.5 to 81.4 × 14.8 to 22.5 µm (n=50) and devoid of any conspicuous fibrosin bodies. Germ tube was, filiform, twisted, arose basally and measured 2.0 to 5.0 µm in width. Chasmothecia were hypophyllous, black, scattered, globose and measured 195.0 to 255.0 µm in diameter (n=50) having 8 to 12 equatorial, acicular, up to 270.0 µm length appendages with 25.9 to 44.4 µm diameter bulbous base (n=50) and obtuse or subacute apex. Asci in a chasmothecium were clavate to saccate, 62.9 to 81.4 × 18.5 to 22.2 µm (n=50), stalked, and two- spored. Ascospores were 33.3 to 40.7 × 12.9 to 18.5 µm (n=50), pale yellowish or golden brown in color. All morphological features were consistent with Phyllactinia pyri-serotinae (Braun and Cook 2012). To confirm the fungus identity at molecular level, DNA of two isolates was extracted from chasmothecia. The internal transcribed spacer (ITS) sequence of ribosomal DNA was amplified with the primers ITS1 and ITS4 (White et al. 1990) and sequenced. The ITS sequences submitted to NCBI GenBank under Accession No. MZ505441 and MZ505442 have 97 (416/427) & 96 (424/440) per cent and 99 (424/430) & 98 (428/438) per cent base pair matching, with that of P. pyri-serotinae isolates from Japan (AB080521 and AB985507), respectively. Thus, the pathogen was identified as Phyllactinia pyri-serotinae Sawada based on morphological and molecular sequence analyses. The pathogenicity tests of both the isolates were carried out on one year old pear saplings (cv. Fertility) and repeated twice. The inoculum was prepared by collecting P. pyri-serotinae conidia in sterile distilled water from infected pear leaves. Three saplings were inoculated by spraying (15ml per sapling) the inoculum (3 x 105 spores ml-1) on leaf surfaces, while same number of saplings sprayed with sterile distilled water served as non-inoculated controls. After 15 days of incubation at 25oC in a green house, similar symptoms as observed on naturally infected plants were observed on inoculated plants and uninoculated plants remained symptomless. The pathogen of interest observed on inoculated plants was morphologically characterized and found to be similar to P. pyri-serotinae. The voucher specimen was deposited in the Herbarium Crytogamae Indiae Orientalis (HCIO), IARI, New Delhi under accession number 52213. Pear is the third most important temperate fruit grown in India (Chattopadhyay 2009) and our study reveal P. pyri-serotinae as the new causal agent of powdery mildew in addition to P. guttata (Dhar and Shah 1982) under Indian conditions.

Optimizing the Agrobacterium tumefaciens-mediated transformation conditions in Colletotrichum lindemuthianum: a step forward to unravel the functions of pathogenicity arsenals.

Colletotrichum lindemuthianum is a hemibiotrophic fungal pathogen that causes bean anthracnose and it is rated among the top 10 important diseases infecting beans. Currently our knowledge on molecular mechanisms underlying C. lindemuthianum pathogenesis is limited. About five pathogenicity genes have been identified in C. lindemuthianum using Restricted Enzyme Mediated Integration and the transformation using Agroinfection has not been optimized. In this study, a series of experiments were conducted to optimize the key parameters affecting the Agrobacterium tumefaciens-mediated transformation for C. lindemuthianum. The transformation efficiency increased with increase in spore concentration and co-cultivation time. However, the optimum conditions that yielded significant number of transformants were 106 ml-1 spore concentration, co-cultivation time of 72 h, incubation at 25°C and using a cellulose membrane filter for the co-cultivation. The optimized protocol resulted in establishment of large mutant library (2400). A few mutants were melanin deficient and a few were unable to produce conidia. To determine the altered pathogenicity, two new approaches such as detached leaf and twig techniques proved reliable and require fewer resources to screen the large mutant libraries in a short time. Among the 1200 transformants tested for virulence, 90% transformants were pathogenically similar to wild type (race 2047), 96 and 24 were reduced and impaired, respectively. The altered avirulent transformants can prove vital for understanding the missing link between growth and developmental stages of pathogen with virulence. This platform will help to develop strategies to determine the potential pathogenicity genes and to decipher molecular mechanisms of host-pathogen interactions in more detail.