Bacterial rots and phytoplasmas in cacti

Soft rot is a set of symptoms

Rapid softening and watery breakdown of fleshy tissues indicate rot, but do not identify a bacterium. A similar appearance may accompany infection by an oomycete or fungus, or secondary colonisation of tissue previously destroyed by frost. Smell, colour and the rate of change provide supporting information. Identifying the cause requires a properly collected sample from the boundary between living and damaged tissue, and laboratory diagnostics. [Mejía-Sánchez et al. 2019] [9-WWW] [Fleischer and Schütz 1978, pp. 185–188]

A documented example of Pectobacterium

In a study of the Mexican cactus described as Neobuxbaumia tetetzo, bacteria were isolated from soft rot symptoms, identified, and the symptoms reproduced by inoculating healthy material. The authors used the name Pectobacterium carotovorum subsp. brasiliense. LPSN now accepts Pectobacterium brasiliense as a separate species. Updating the bacterium's name does not change the host or experimental conditions. Nor does it prove that every wet rot of a household cactus has the same cause. [Mejía-Sánchez et al. 2019] [16-WWW]

This example corrects the statement in Fleischer and Schütz's handbook that bacteria are essentially not causes of cactus diseases. The historical opinion is treated as a record of the authors' knowledge and experience at the time, rather than a current diagnostic rule. [Mejía-Sánchez et al. 2019] [Fleischer and Schütz 1978, p. 185]

Phytoplasmas. Bacteria with a different way of life

Phytoplasmas are bacteria without a cell wall, associated with the host's vascular tissue. They may accompany developmental disorders rather than cause typical wet rot. In the study by Omar and colleagues, sequences of group 16SrII phytoplasmas were detected in fasciated succulents. One host belonged to Cactaceae. The other succulents studied were not cacti. [Omar et al. 2014]

Diagnosis requires molecular tests with appropriate controls. The study's findings and the limits of causal inference are discussed in the chapter on cristata forms. [Omar et al. 2014]

Management when bacterial disease is suspected

The plant is separated from the others, water splashing and sap transfer on tools are limited, and the boundary of the changes is documented. A rapidly disintegrating specimen is removed from use for cuttings and scions. When damage is extensive, removing the plant and contaminated material limits the source of infection more reliably than repeated, non-specific sprays. [9-WWW] [Hinz and Abramowicz 1986, pp. 102–108]

Attempting to save a healthy-looking fragment does not confirm that it is pathogen-free. A new cut must be assessed for the extent of changes, and tools and the work area cleaned before handling the next plant. Recommendations to use antibiotics are not adopted from books, nor is it assumed that every copper product is currently authorised for such use on cacti. The label of the specific product defines its current uses. [9-WWW] [10-WWW] [Fleischer and Schütz 1978, pp. 185–188]

Klebsiella aerogenes in Austrocylindropuntia subulata

Basavand and colleagues described wet, watery stem rot of Austrocylindropuntia subulata in Iran. Isolates were identified as Klebsiella aerogenes from microbiological characteristics and rpoB sequences. Infection of healthy plants produced symptoms, from which the bacterium was isolated again. This provides evidence of the cause in the case studied. The work did not assess plant protection products or treatment regimens. [Basavand et al. 2020]