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Pathogen Biology

Phytophthora capsici is classified in the family Pythiaceae, order Peronosporales, and class Oomycetes. Oomycetes are not true fungi and have been placed in the kingdom Stramenopila. They are more closely related to brown algae than to true fungi. The pathogen produces asexual sporangia and biflagellate zoospores and sexual oospores. Mycelia are coenocytic (non-septate). Phytophthora capsici grows at 10 to 36°C (50 to 97°F), with optimal temperatures of 24 to 33°C (75-91°F). This pathogen grows rapidly on lima bean agar (Figure 12), and the colony diameter can reach up to 8 cm (3 in.) in 5 days. The growth patterns of colonies can vary from cottony, petaloid, rosaceous, to stellate (star-shaped) (Figure 12).

Figure 12.

Sporangia (asexual fruiting bodies) of P. capsici are produced on sporangiophores (sporangia-producing hyphae) and are mostly papillate (having a small rounded protuberance). Sporangial shapes are influenced by light and other cultural conditions, and may appear as sub-spherical, ovoid, obovoid, ellipsoid, fusiform, or pyriform. The lengths and widths of sporangia can vary from 32.8 to 65.8 and 17.4 to 38.7 μm, respectively. Length/width ratios of sporangia range from 1.3:1 to 2.1:1. Sporangia have long pedicels (stalks), ranging from 35 to 138 μm (Figure 13). Pedicellate sporangia can be dispersed in wind driven rain. Under moist conditions, zoospores (asexual spores) are produced inside sporangia. Zoospores are single-celled and biflagellate. Phytophthora capsici also produces chlamydospores (thick-walled asexual spores), which may be terminal or intercalary (between cells) on the mycelium. Chlamydospores can range in diameter from 22 to 39 μm.

Figure 13.

Phytophthora capsici produces sexual structures called antheridia and oogonia, and sexual spores called oospores (Figures 14 and 15). Phytophthora capsici is predominantly heterothallic with two mating types known as A1 and A2. Antheridia are amphigynous (forming a collar at the base of the oogonium after the young oogonium grows through it), with diameters of 12–21 to 12–17 μm. Oogonia are spherical or sub-spherical, with diameters ranging from 23 to 50 μm. Oospores are predominantly plerotic (filling the oogonium) with wall thicknesses ranging from 2 to 6 μm, and diameters ranging from 22 to 35 μm.

Figure 14.

Figure 15.

Phytophthora capsici is distinguished from other Phytophthora species by its sporangial morphology. Sporangia of P. capsici are caducous (easily separated from sporangiophores), have long pedicels, and are spherical to elongate with a tapering base.

Significant differences in virulence (degree of pathogenicity) and genetics among isolates of P. capsici have been reported(Figure 16). Several methods can be used to study the genetic variation of P. capsici and other fungi. Sequencing and/or restriction digest of internal transcribed spacers (ITS) regions can be used for species identification. A specific PCR primer (Pcap) has been developed that can be used with iTS primers to specifically amplify P. capsici. Inter-simple sequence repeats (ISSR) amplification, amplified fragment-length polymorphism (AFLP), allozyme genotyping, and restriction fragment length polymorphisms with a probe can be used to study genetic variation among populations of P. capsici.

Figure 16.