Pectobacterium spp. and Dickeya spp. cause aerial stem rot on potatoes worldwide (Charkowski, 2018). Potato plants (cv. Xisen6# or Youjin) with aerial stem rot or blackleg symptoms (Fig. S1) were o...

The effects of drought stress on phospholipase D (PLD) gene expression and enzymatic activity were investigated in ‘Pineapple’ sweet orange. PLDs are tissue-specific, with overlapping functions, and in response to stress they may interact with ABA signaling. Tissue specificity for expression of PLDs and their regulation are unknown in citrus. To assess PLD response to water stress and gene expression/regulation in citrus, we subjected potted seedlings to increasing levels of soil drought. Evapotranspirational demand (ET) was estimated by measuring weight loss in pots, and water stress was further assessed by measuring ABA content. Three treatments were performed over a 3-week period: (a) Control treatment without drought stress (100% of daily ET); (b) mild water stress (50% ET); and (c) severe water stress (0% ET). ABA content increased during drought stress in both roots and leaves, being higher in leaves than in roots by the end of the experiment for any stress condition assayed. PLD enzymatic activity was monitored and expression of five PLD genes was studied. PLD activity increased linearly over time in response to increased soil drought and was around three times higher in roots than in leaves. PLD activation occurred initially in roots and then in leaves. PLD gene expression in response to stress soil drought differed between roots and leaves. These results show the potential of PLD as a suitable indicator of stress severity in citrus.

Currently, little is known about the host pathogen interaction between Phytophthora spp. and citrus roots versus leaves. Therefore, we compared the molecular events occurring in citrus roots and leaves after zoospore inoculation with Phytophthora nicotianae. We analyzed the physical characteristics and genetic responses to P. nicotianae infection of leaves and roots for susceptible and tolerant citrus rootstocks to examine the potential for leaves to model root responses to P. nicotianae infection. Leaves responded faster and stronger to P. nicotianae infection than roots, and leaves showed greater differential response than roots. In addition to differences in hormonal responses, sugar, phospholipase D (PLD), and phospholipase A (PLA) involvement in the interaction between citrus and P. nicotianae was identified. This work, for the first time, creates a solid P. nicotianae zoospore infection protocol, reports P. nicotianae infection on citrus leaves through stomata, and provides evidence that different host organs respond to the pathogen differentially in timing and magnitude. This work identifies the hormones, sugars, pathogenesis-related genes, PLDs, and PLAs that are involved in the molecular events occurring in citrus under infection of P. nicotianae zoospore, and advances our understanding of the mechanisms underlying the interaction.

Potato (Solanum tuberosum L.) common scab can be caused by multiple pathogenic Streptomyces spp. worldwide. Potato tubers (cv. Favorita) with severe pitted common scab symptoms were observed at a small farm (2 hectares) during harvest in Anshun, Guizhou province in early May 2020. The disease incidence was around 10%, and symptomatic samples were collected to isolate the pathogen. Two isolates, ZR-IMU141 and ZR-IMU146 (Accession number MW995958 and MW995959 respectively), showed more than 99% sequence identity to S. stelliscabiei sequences (Accession No. HM018085). Five house-keeping genes for multi-locus sequence analyze (MLSA) of Streptomycetaceae were amplified, sequenced and uploaded to NCBI: atpD (MZ343164 and MZ343165), gyrB (MZ343162 and MZ343163), recA (MZ343166 and MZ343167), rpoB (MZ343168and MZ343169) and trpB (MZ343170 and MZ343171). All the genes show over 98% identity with S. stelliscabiei. Phylogenetic trees of 16S rRNA gene sequence and multi-locus sequence analysis (MLSA) were constructed. The two isolates contain pathogenicity genes txtAB, nec1, and tomA, which was confirmed by PCR. To complete Koch's postulates, 9 potato seedlings (cv. Favorita, 15 centimeters high), were transferred to new pots and inoculated with spore suspensions of ZR-IMU141 and ZR-IMU146 (10⁴ CFU/ml), or water as a negative control. Two months later, potato tubers inoculated with either ZR-IMU141 or ZR-IMU146 exhibited typical symptoms of potato common scab, such as superficial or deep, raised, pitted, or polygonal lesions like the field symptoms, but the negative controls remained asymptomatic. The pathogens were reisolated from the lesions and confirmed identical to the original isolate by 16s rRNA gene sequences. To our knowledge, this is the first report of S. stelliscabiei causing potato common scab in Guizhou province, China. We believe that this report will draw attention to the study and management of the increased pool of scab pathogens in China.