Abstract
The degree of sporangium caducity (expressed as percentage detached sporangia) and sporangium pedicel length were evaluated comparatively in 16 isolates representing 10 species or morphological forms (MF) of
Phytophthora for their diagnostic values in identification.
The percentage of detached sporangia differed between species and varied also among isolates within the same species or morphological form, ranging from 3 · 8 % to 58 · 5 % among the 16 isolates tested. The two non-caducous species,
P. cinnamomi Rands and
P. parasitica Dast., produced low percentages of detached sporangia; each species carried a hyphal attachment of irregular length or was without hyphal attachment of any length.
Phytophthora cactorum (Leb. & Cohn) Schroet.,
P. colocasiae Raciborski, and ‘
P. palmivora’ (MF3), under these conditions, also produced low percentages of detached sporangia but each possessed pedicels of uniform length. Age of the sporangium-bearing culture greatly affected the degree of caducity but not the pedicel length in the taxa tested. This indicates that only mature sporangia with uniform pedicel lengths are detached and that the low percentages of sporangium detachment in some caducous species may be due to high proportions of immature sporangia in test cultures. On the basis of high degrees of uniformity in pedicel length within majority populations of detached sporangia, the following were designated or confirmed as caducous species and grouped into three categories based on average pedicel lengths. They were (
a) with short pedicels (< 5 μm):
P. cactorum, P. infestans (Mont.) deBary, and
P. palmivora (Butl.) Butl. (MF1); (
b) with pedicels of intermediate lengths (5–20 μm): {itP. botryosa Chee,
P. Colocasiae, and ‘
P. palmivora’ (MF3); and (
c) with long pedicels (> 20μm):
P. hibernalis Carne and ‘
P. palmivorars (MF4). It is concluded that the degree of sporangium caducity (i.e. high or low percentage detachment) is of questionable diagnostic value and cannot, by itself, be used to determine sporangium caducity in most
Phytophthora species or isolates, unless uniformity in pedicel length is also considered. Pedicel length is a much more stable sporangium character having high diagnostic value in identifying
Phytophthora species. A species should be considered caducous only if a certain percentage of sporangia are detached at maturity and if the detached sporangia each carry a pedicel of uniform length.