Stalk length as a taxonomic criterion for Phytophthora palmivora isolates from cacao

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Abstract
Eighty cacao isolates of Phytophthora from Africa, the Americas, the Pacific, and Southeast Asia, originally designated as Phytophthora palmivora, were studied. The sporangial stalk or pedicel length of various isolates was significantly different. These isolates were separated into four groups on the basis of the stalk or pedicel length. The differences in characteristics among the four groups and their relationships to the morphological forms (MF) of P. palmivora are described.One group of isolates, designated as MF 1, had short and thick sporangial stalks averaging less than 5 μm in length. Isolates in a second group corresponded to MF 3, with rather thin sporangial stalks, intermediate in length, averaging 5–15 μm. Sporangial stalks of a third group of isolates, designated as MF 4, had unusual and characteristically long stalks with an average length of more than 15 μm. A fourth group of isolates had noncaducous sporangia. Similarities between MF 4 isolates and the type culture of P. capsici are noted.The stability of sporangial stalk length under several different environmental conditions makes this character very useful as a taxonomic criterion for identifying isolates in the P. palmivora complex.

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