Introduction

Phytophthora megasperma  Drechsler (1931)

Phytophthora megasperma was isolated by Drechsler in 1931 from root tissue of hollyhock (Althaea rosea) (Drechsler, 1931). A synonym of P. megasperma is Pythiomorpha miyabeana S. Ito & Nagai (1931) (Cline et al., 2008). Tompkins et al. (1936) broadened Drechsler’s description to include isolates with small oogonia from cauliflower. Waterhouse (1963) separated P. megasperma into P. megasperma var. sojae and P. megasperma var. megasperma based on oogonium size (small versus large, respectively). Kuan and Erwin (1980) questioned the separation of the species based on oogonium size since isolates from different hosts show a continuum of oogonium sizes, and they renamed the species P. megasperma f. sp. glycinea and P. megasperma f. sp. medicaginis for the soybean and alfalfa isolates, respectively. Others also revised the species (Faris et al., 1989). Hansen and Maxwell redescribed P. megasperma in 1991 based on Drechsler’s original description (Hansen and Maxwell, 1991). Isolates of P. megasperma from different hosts were previously given subspecies distinction but are now designated as three separate species, known as P. sojae (soybean), P. medicaginis (alfalfa), and P. trifolii (clover) (Hansen and Maxwell, 1991). The broad host range group of P. megasperma isolates from fruit trees (e.g., cherry, apple) and from the DF1 and soybean race nonclassifiable isolates have recently been separated into the new species P. rosacearum and P. sansomeana, respectively (Cooke et al., 2000; Förster and Coffey, 1993; Hansen et al., 2009). Gallegly and Hong (2008) separate P. megasperma into three subgroups based on the absence (I, II) or presence (III) of hyphal swellings and chlamydospores, oogonial and sporangial size and shape, antheridial characters, and DNA markers. Type II isolates most closely fit the original description (Gallegly and Hong, 2008). See Erwin and Ribeiro (1996) for a thorough explanation of the nomenclature of this complex group. P. megasperma is a group V Phytophthora species (Stamps et al., 1990) (Fig. 1).

Cultural Characteristics

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores are simple or sparingly branched, thin, and 2–2.5 µm wide. At the base of the sporangium, sporangiophores can reach widths of up to 5 µm. Sporangiophores usually proliferate inside the empty sporangium (Waterhouse and Waterston, 1966).

Sporangia:

Sporangia are nonpapillate, noncaducous, ovoid, obpyriform, and persistent on the stalk and they proliferate internally (Bush et al., 2006; Drechsler, 1931). Sporangia are 25–45 × 35–60 µm (Fig. 3).

Chlamydospores:

P. megasperma type I and II isolates do not produce chlamydospores. Type III isolates produce chlamydospores.

Hyphae:

Hyphae are 3 µm wide (Waterhouse and Waterston, 1966). Hyphal swellings are rare on solid media but are produced in aqueous cultures of some type III isolates (Gallegly and Hong, 2008). Swellings are rounded or angular and can occur in chains or clusters (Fig. 1).

Sexual Structures

P. megasperma is mostly homothallic, but Barr (1980) and Ho (1986) reported that a large-oospore isolate from alfalfa was heterothallic.

Antheridia:

Antheridia are mainly paragynous, but some amphigynous antheridia form (type I isolates) (Gallegly and Hong, 2008). Antheridia are irregularly spherical or ellipsoid and 10–18 x 14–20 µm.

Oogonia:

Oogonia are produced in both host tissues and axenic cultures. Oogonia are smooth, subspherical, and 42–52 µm in diameter (average 47.4 µm). The oogonia wall is colorless or yellowish, smooth, and up to 1.5 µm thick (Drechsler, 1931). Type II isolates produce the largest oogonia (Gallegly and Hong, 2008).

Oospores:

Oospores are 26–52 µm in diameter (average 41.1 µm) (Drechsler, 1931) (Fig. 4). The oospore wall is smooth and up to 5 µm thick (unstained) or 7 µm thick (stained).

Host Range and Distribution

The species has a wide host range and can infect many different plant species (Hansen and Maxwell, 1991).

Symptoms

Root Rot of Brassica spp. (e.g., Cabbage, Cauliflower, Brussels Sprouts, Turnip, Kale, and Rutabaga):

Infection of the plant starts at the roots and then spreads to the leaves. Leaves turn progressively purple or reddish from the margins inward, wilt, and eventually shed. Stems become soft and flaccid, and the lower end of the taproot blackens and rots. The cortex sloughs off the taproot and lateral roots, and adventitious roots often develop above the diseased portions of the taproot. The disease develops in moist soil at moderate soil temperatures of 20–30°C.

Crown Rot of Malus pumila (Apple):

Crown rot symptoms include delayed bud break, stunted growth, orange discoloration of the bark, and reduced crop yield (Robertson and Dance, 1971). Crown rot produces cankers that eventually girdle the trunk and cause the bark within to rot completely. Disease symptoms are similar on Prunus spp. (Figs. 5 and 6), except that gum exudes from the cankers. Lesions of various lengths develop on lateral roots, bud break is delayed, and branches die back (Erwin and Ribiero, 1996). The disease develops after prolonged heavy rains.

References