First Report of Root Rot Caused by Pythium ultimum on Acanthosicyos horridus (Cucurbitaceae).

Acanthosicyos horridus in the family Cucurbitaceae is a perennial, dioecious phraetophyte with reduced leaves, native to the Namib Desert in Africa, where it plays a crucial role in the ecosystem (Klopatek and Stock 1994). In the summer of 2015, damping-off symptoms were observed on seedlings and young plants of A. horridus in research plots in Charleston, SC. Within 4 weeks after transplanting, the incidence of the 36 affected plants reached 58.3%. The observed symptoms included browning of the roots and the hypocotyl, wilting, and ultimately plant death. Roots of symptomatic plants were placed onto water agar amended with 0.1 g/liter streptomycin sulfate. Eight isolates of similar morphology were recovered from symptomatic plants, and hyphal tipped to obtain pure cultures. Two of the isolates were characterized further. A greenhouse pathogenicity test was done as described by Chellemi et al. (2000) to confirm Koch’s postulates. Wheat grain inoculum of each isolate was incubated in the dark for 12 days and shaken daily. Peat-based potting soil (150 g) and 1.25 g of wheat grains from each isolate was added to each of the pots, and subsequently planted with 2- to 3-week-old seedlings. Five pots were prepared as negative controls with uncolonized grains. Pots were kept at 30 ± 2°C in the greenhouse for 28 days and watered regularly. The test was repeated once. After 21 days, symptoms similar to those observed in the field appeared, except that the plants did not die during pathogenicity testing. In both experiments, the pathogen was recovered from all roots and stems of inoculated plants, but never from control plants. For production of sexual structures, the two isolates were transferred onto Super V8 agar (Jeffers and Aldwickle 1988) and incubated at 20°C in the dark for 18 days, after which structures were examined at 400× magnification. Oogonia were globose, nearly plerotic, with a mean diameter of 18.6 µm (std. dev. = 1.2 µm; n = 20). Oospores were globose with a mean diameter of 15.8 µm (std. dev. = 1.3 µm; n = 20). To examine asexual structures, petri dishes containing sterile deionized water, autoclaved St. Augustine grass (Stenotaphrum secundatum) blades, and agar pieces of the cultures were prepared separately for each isolate as described by Njoroge et al. (2008). After 4 days, mycelial growth was examined at 400× magnification. Both isolates readily produced (sub)globose intercalary and terminal sporangia with an average diameter of 20.8 µm (std. dev. = 2.2 µm; n = 20). Both isolates were identified through sequencing of the ITS region using ITS4/5. The sequences of the 801-bp fragment were identical, and showed 99% identity to the ITS sequence of the neotype specimen of P. ultimum var. ultimum (accession number AY598657). The fragments were deposited in GenBank under accession numbers KU672382 and KU672383. Since P. ultimum is a species complex that is currently undergoing taxonomic revision, the pathogen is referred to as P. ultimum without reference to variety (Robideau et al. 2011). This is the first report of Pythium damping-off and root rot caused by P. ultimum on A. horridus. © The American Phytopathological Society.