Phytophthora infestans is the causative agent of late blight and one of the most economically significant diseases of Solanaceous crops. Detached leaf assays are useful tools to efficiently assess how differing environmental conditions and management controls may affect the infection process and overall disease severity. However, these protocols need refinement to more fully reflect natural infection processes in the field. In a detached leaf assay experiment to study the infection rate and the development of late blight as affected by initial sporangia concentration, greenhouse-grown tomato (Solanum lycopersicum), potato (Solanum tuberosum), and eggplant (Solanum melongena) were used. The study showed a clear dose-dependent response: high concentrations of sporangia (50,000/ml and 10,000/ml) caused rapid and extensive spot formation, while at low concentrations (2,000/ml), symptoms were mild and control leaves remained healthy. The spread of the spots and the percentage of the affected area were assessed using the ImageJ program for 7 days. The results confirm the effectiveness of the detached leaf method for quantifying pathogen aggressiveness and provide insight into the early stages of host-pathogen interaction.

Bacterial soft rot of potato is predominantly associated with members of the genera Pectobacterium and Dickeya, whereas the contribution of other bacterial taxa to the etiology of the disease remains insufficiently characterized. This study aimed to characterize a bacterial isolate recovered from potato tubers exhibiting naturally occurring soft rot symptoms in Kazakhstan and to evaluate its pathogenic potential. Twelve bacterial isolates were obtained from symptomatic tissue, of which only isolate ZA4 reproducibly caused pronounced tissue softening and maceration following wound inoculation of healthy tubers, whereas disease development remained minimal in the negative control. Reproduction of characteristic symptoms and subsequent reisolation of the bacterium from experimentally affected tissue fulfilled the principal criteria of Koch’s postulates. Multilocus phylogenetic analysis based on 16S rRNA, gyrB, rpoB, and pyrE sequences assigned isolate ZA4 to Bacillus altitudinis. When tested across five potato cultivars and one commercial sample of unknown cultivar identity, the proportion of macerated tissue ranged from approximately 60–85% at 24 h and varied among the tested materials, increasing to 95–98% at 48 h, indicating rapid progression of tissue maceration under the bioassay conditions. ZA4 also exhibited pectinolytic, cellulolytic, proteolytic, and amylolytic activities. These findings demonstrate that B. altitudinis ZA4 is capable of inducing pronounced tissue maceration and soft-rot symptoms in wounded potato tubers under artificial wound inoculation conditions. However, broader studies encompassing a larger number of samples and a wider geographic area are required to determine the occurrence and epidemiological significance of B. altitudinis in association with potato soft rot in Kazakhstan.