Advances in Crop Science and Technology
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Polylepis sp. (Rosaceae) is a native plant of the Andean region of Bolivia. Taxonomically, Polylepis is a complex genus consisting of some 20 species distributed throughout the Andes [1]. In 2015, Polylepis plants (P. glomeratus, P. lanata and P. racemosa ) growing in a nursery in the Pajchanti locality (Ayopaya Province, Department of Cochabamba, Bolivia) were reported attacked by the Oomyete Peronospora sp. [2]. Earlier, in 2009, Polylepis racemosa plants growing in a nursery at an altitude of 2650 m in the locality of Palca (Independencia Province, Department of Cochabamba, Bolivia) were reported to have a severely destructive leaf disease (Figures 1A and 1B).

Figure 1: (A) Nursery production of Polylepis (local native name=Quehuiña) in the Andean locality of Palca (altitude 2650 m) affected by mildew, and (B) Plants affected by disease plus dead, and still healthy plants.

The presence of the disease (in Palca community) was generalized and showed different degrees of severity. The leaves of the infected plants initially turned chlorotic and later spots appeared. Eventually the plants became defoliated (Figures 2A and 2B). The underside showed clear signs of a fungus similar to downy mildew (Figure 3A). Observations made under the microscope revealed conidiophores branching dichotomously 4-5 times (length 250-500 μm; mean 350 μm) and sterigmata bearing a single, grayish, round or slightly ovoid conidium (Figure 3B) some 18-24 long and 16-22 μm wide (Figure 3A and 3B). This description approaches that of P. sparse or the P. oblatispora group [3,4] Oospores were not found. Pathogenicity tests were performed twice under controlled conditions (18ºC-20ºC, 80%-100% RH) by inoculating leaves of 30 day-old healthy P. racemosa plants with a conidial suspension (approx. 105 conidia/ml). Noninoculated plants were sprayed with H2O to serve as controls. After ten days, symptoms developed only in the inoculated plants, and Peronospora -type conidiophores were observed developing on the underside of the leaflets (Figure 3B).

Figure 2: (A) Symptoms of Peronospora sp. infection on P. racemosa , (B) On individual leaflets, and (C) Initial symptoms as seen under magnification.

Figure 3: (A) Peronospora sp. downy mildew on the underside of the leaves (red circle), and (B) Conidiophore with conidia at 400X magnification.

Downy mildews are a notorious group of oomycete plant pathogens, causing important economic losses in different crops and ornamental plants [5]. Peronospora is known to affect families such as the Fabaceae, Caryophyllaceae, Ranunculaceae and Boraginaceae, and has recently been described affecting Rosaceae species [3,5] but never Polylepis. This is the first description of a Peronospora sp. on P. racemosa in the Andean highlands of Bolivia.

The author thanks Gualberto Mamani, forestry student at the Escuela de Ciencias Forestales (ESFOR), Universidad Mayor de San Simon, Cochabamba, Bolivia, for help with the collection of samples and fieldwork, and Dr. Young-Joon Choi, Assistant Professor in the Department of Biology, College of Natural Sciences, Kunsan National University, South Korea, for useful comments on morphology and identification.