Q What causes the brown, scablike lesions I’m seeing on my Kalanchoe thyrsifolia and sedum?
A This may surprise you, but it is most likely powdery mildew. Unlike the typical talcum-like leaf spots, powdery mildew infection on succulents causes brown, scabby lesions, generally limited to one side of the leaf.
While environmental conditions remain favorable for infection and disease development (warm days and cool nights), the size and number of lesions will continue to increase, resulting in large patches of brown necrotic tissue. Under high humidity, wispy fans of white fungal mycelium can sometimes be seen within or at the edges of the scablike lesion (the mycelium advances from the initial lesion to infect adjoining leaf tissue), but most of the time, all you will observe is a more or less dry-looking, scabby lesion. Unfortunately, the lesions are irreversible, and this can impact the marketability of the crop. Therefore, early detection and treatment are necessary to minimize future losses.
Q Is there a difference in efficacy between Aliette and its generic forms?
A There are several phosphonate fungicides that may be considered generic forms of Aliette. That may sound a little confusing at first, given that the common chemical name for Aliette is fosetyl-aluminum. When fosetyl-aluminum is applied, the plant absorbs it and converts it to phosphonate and, ultimately, phosphite ions, which are responsible for the antifungal activity. The primary difference between Aliette and other phosphonate fungicides is the source of the phosphite ions.
In addition to Aliette, phosphonate fungicides labeled for ornamental use include Biophos (dipotas-sium phosphonate and dipotassium phosphate); Alude, Fosphite, K-Phite 7LP, Reliant, Resyst (mono- and dipotassium salts of phosphorous acid); Vital (potassium phosphite); and Magellan (mono- and dibasic sodium, potassium and ammonium phosphites).
Phosphonate fungicides are active against Pythium, Phytophthora and the closely related downy mildews. Several phosphonate fungicides have been evaluated in efficacy trials across the country, but as with all fungicide trials, results can vary depending on disease pressure, plant host, strain of pathogen present and the researcher conducting the study. As such, it is important to look at overall trends for a particular fungicide product.
Both Aliette and Biophos gave consistent control of Phytophthora root rot or blight caused by Phytophthora nicotianae for most host plants evaluated. Vital and Alude also performed well in efficacy trials for control of P. nicotianae. Aliette, Alude, Biophos, Magellan and Vital were demonstrated to have good to excellent control of Phytophthora root rot caused by Phytophthora cinnamomi.
However, results were not consistent with all host plants or Phytophthora species evaluated. In my own efficacy trial for management of coleus downy mildew, Aliette and Vital both significantly reduced the number of infected leaves and amount of sporulation compared to the untreated, inoculated controls. Overall, Aliette and its generic forms have demonstrated good efficacy. It is important to keep in mind that phosphonate fungicides should be applied preventively.