302 M. BABADOOST
globose, terminal or intercalary, and 26–32μm in diameter. Usually,
three to six antheridia per oogonium are produced. Stalks are branched,
are diclinous or occasionally monoclinous, originate at various dis-
tances below the oogonium, and mostly measure 8–30 ×4–8μm.
Oospores are aplerotic, colorless or yellowish, and 20–27 μmin
diameter.
- Host Range. Pythiumspecies cause infection on a wide range
of plant species, belonging to different families, including Ama-
ranthaceae, Amaryllidaceae, Araceae, Basellaceae, Bromeliaceae,
Cactaceae, Chenopodiaceae, Compositae, Coniferae, Convolvulaceae,
Cruciferae, Cucurbitaceae, Euphorbiaceae, Gramineae, Leguminosae,
Linaceae, Malvaceae, Moraceae, Passifloraceae, Rosaceae, Solanaceae,
Umbelliferae, Violaceae, Vitaceae, and Zingiberaceae (Waterhouse and
Waterston 1964; Gubler and Davis 1996b; Parveen and Sharma 2015).
Numerous plant species are hosts ofPythiumpathogens. - Disease Cycle and Epidemiology. Pythiumspecies are soil habi-
tant, existing as nonspecialized parasites. Oospores are overwinter and
oversummer structures ofPythiumspecies (Rahimian and Banihashemi
1979; Gubler and Davis 1996b). Sources of inoculum ofPythiumspecies
may be contaminated potting media (Moorman et al. 2002); irrigation
water, particularly in flood irrigation in which drainage times are long
(Sanogo and Moorman 1993; Hong and Moorman 2005); and organic
debris, soil, and tools (Stephens et al. 1983; Moorman et al. 2002). Some
Pythiumspecies also can spread by insects and fungus gnats (Goldberg
and Stanghellini 1990; Hyder et al. 2009).
Infection by Pythium species takes place through wounds or at the
site of fruit contacting soil.Pythiumspecies grow intracellularly and
produce fluffy, white aerial mycelia on the surface of decayed tissue.
Secondary cycles of the pathogens are influenced by soil moisture con-
ditions. Conditions favorable for disease development differ according
to the pathogen species. For example,P. ultimumrequires cool, wet soils
for disease development, whereasP. aphanidermatumandP. myrioty-
lumrequire higher soil temperatures (32–37◦C), and foliar symptoms
also are exacerbated by hot weather and low relative humidity.P. irreg-
ulareappears to be the least virulent species on cucurbits.
When soils are wet, infection of seedling roots occurs soon after radi-
cle emergence. Mature plants show symptoms on feeder roots, lateral
roots, and the taproot and hypocotyl. It has been shown that within 2 d
following irrigation,Pythiumspecies can be recovered from feeder roots