Rots, damping-off and leaf or stem spots. Fungi and oomycetes

Fungus, oomycete and disease symptom

Older guides discussed Phytophthora and Pythium together with fungi. Oomycetes are now distinguished from true fungi. Similar filamentous growth and root symptoms do not imply a close taxonomic relationship or equal sensitivity to plant protection products. The names “damping-off”, “root rot” and “stem spot” describe sets of symptoms, rather than identification of the causal organism. [Cho et al. 2016] [Fleischer and Schütz 1978, pp. 186–188]

Fusarium and Neocosmospora

Kamali-Sarvestani and colleagues studied rots of cacti and other succulents in greenhouses around Shiraz. Identification included morphological traits, sequence data and pathogenicity tests. They demonstrated the involvement of Fusarium oxysporum f. sp. opuntiarum, F. proliferatum and Neocosmospora falciformis. The last name reflects a classification that separates Neocosmospora from Fusarium in the broad sense. The name Fusarium falciforme also appears in the literature. [Kamali-Sarvestani et al. 2022]

Pathogenicity and susceptibility varied among isolate–host combinations. The result applies to the host–isolate combinations studied. The ability to survive in plant debris, the broad host range of some isolates and vegetative propagation were epidemiologically important. The mere presence of a fungus in a dead fragment does not establish that it initiated the disease. [Kamali-Sarvestani et al. 2022]

Bipolaris and the historical Helminthosporium

The names Helminthosporium cactivorum and Drechslera cactivora appeared in older cactus literature. Cho and colleagues discuss this causal organism as Bipolaris cactivora and compare it with other rot pathogens. Retaining historical names in the commentary makes it possible to locate the old description. It does not justify diagnosing the disease from a dark spot alone. [Cho et al. 2016] [Fleischer and Schütz 1978, pp. 186–187]

Phytophthora, Pythium and damping-off

Oomycetes associated with roots and the stem base are important in the differential diagnosis of wilting and seedling death. In a study by Cho and colleagues, PCR was developed to distinguish selected cactus pathogens, including Phytophthora cactorum and P. nicotianae. The method had a defined set of diagnostic targets. It was not a universal test for all causes of rot. [Cho et al. 2016]

In seed sowing, the onset of changes must be monitored, including root loss, constriction at the base, seedling collapse and expansion of the affected area. Moist conditions are needed for germination, but contaminated material, stagnant water and an overheated container can combine to favour disease. Seed, tool and substrate hygiene and prompt removal of affected plants form part of management. Preventive spraying with an arbitrary product does not replace them. [Fleischer and Schütz 1978, pp. 186–188] [Hinz and Abramowicz 1986, pp. 102–108]

Spots, surface mould and dieback

A dark spot, surface growth or hardening necrosis may have different causes. Sooty mould fungi can grow on insect honeydew without being the primary cause of stem damage. Grey mould and other fungi can colonise dying flowers and weakened tissues. Assessment considers whether a change affects only the surface or extends deeper, and whether it remains active. Flower remnants are removed carefully, preserving living areole tissues. [Fleischer and Schütz 1978, pp. 186–188] [Seneta b.d., pp. 108–111] [1-WWW]

Control based on diagnosis

Management begins by isolating the diseased plant, limiting transfer of substrate and water, and improving root conditions. With rapidly progressing or recurrent dieback, testing the material before another treatment is appropriate. Drying does not guarantee pathogen elimination, and a healthy-looking cutting tip is not reliably infection-free material. [Kamali-Sarvestani et al. 2022] [Fleischer and Schütz 1978, pp. 186–188]

Fungicide selection must match the identified causal organism and current label. For example, the checked Magnicur Energy label includes specified uses against Phytophthora disease and diseases caused by Pythium in ornamental plants. This does not make the product a universal treatment for Fusarium, bacteria or viruses. Historical mixtures, doses and recommendations to keep diseased cacti in a refrigerator have not been carried over into current instructions. [10-WWW] [Fleischer and Schütz 1978, pp. 186–188]