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Genus wide phylogeny for Phytophthora using four mitochondrial loci (cox2, nad9, rps10 and secY; 2,373 nucleotides). Maximum likelihood branch lengths shown. Numbers on nodes represent bootstrap support values for maximum likelihood (top), maximum parsimony (middle) and Bayesian posterior probabilities as percentages (bottom). Nodes receiving significant support (>95%) in all analysis are marked with an asterisk (*). Scale bar indicates number of substitutions per site.(Martin, Blair and Coffey, unpublished).
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Phytophthora drechsleri Tucker 1931 (Oomycetes, Pythiales)
Phytophthora erythroseptica var. drechsleri (Tucker) Sarej. 1936
= Pythium teratosporon Sideris 1932 Note: Synonymy based on Erwin & Ribeiro 1996, see also Waterhouse 1970.
Notes: Ho and Jong (1986) listed various synonyms as part of their broad species concept. They considered Phytophthora cryptogea to be synonymous with Phytophthora drechsleri Cooke et al. (2000) found the two species to be distinct based on molecular analysis. Phytophthora melonis and Phytophthora sinensis were also found to be distinct from P. drechsleri. Cooke (2000) also found that P. drechsleri is not closely related to P. cajani which should be considered a distinct species and not a f. sp. or variety of P. drechsleri.
Distribution: Cosmopolitan.
Substrate: Roots, stems, leaves, buds, flowers, fruits, tubers, bark of trunks.
Disease Note: Primarily a root pathogen but also attacks ripening fruit for various crops. Root rot, bark canker fruit rot, stem rot, seedling blight; see Erwin & Ribeiro (1996) for disease symptoms listed by host.
Host: 113 genera in 40 families.
Supporting Literature:
Cooke, D.E.L., Drenth, A., Duncan, J.M., Wagels, G., and Brasier, C.M. 2000. A molecular phylogeny of Phytophthora and related Oomycetes. Fungal Genet. Biol. 30: 17-32
Erwin, D.C., and Ribeiro, O.K. 1996. Phytophthora Diseases Worldwide. APS Press, St. Paul, Minnesota, 562 pages.
Ho, H.H., and Jong, S.C. 1986. A comparison between Phytophthora cryptogea and P. drechsleri. Mycotaxon 27: 289-319
Stamps, D.J. 1985. Phytophthora drechsleri. C.M.I. Descript. Pathog. Fungi Bact. 840: 1-2
Waterhouse, G.M. 1970. The genus Phytophthora de Bary. Mycol. Pap. 122: 1-59
Updated on Jun 06, 2006
P. drechsleri is classified in group VI (Stamps 1985j; Stamps et al. 1990). Figure 1 shows its morphology. See Tables 4.2 and 4.3 for tabular keys and Appendix 4.9 for a dichotomous key (Ho 1992) in Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996). Classical differences between P. drechsleri and P. cryptogea are given in Chapter 20, Table 20.2 (P. cryptogea) in Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996). Morphologically, P. drechsleri and P. cryptogea are similar and differ mainly by the ability of P. drechsleri to grow at 35oC compared with P. cryptogea, which does not (Tucker 1931); however, differentiating these species has become one of the most vexing taxonomic problems in Phytophthora (Erwin 1983; Mills et al. 1991). Ho and Jong (1986) cite several reports that attest to the confusion and lack of agreement in the literature on the identification of isolates as either P. cryptogea or P. drechsleri. To indicate the uncertainty that exists, Keim et al. (1979) described a root rot of Washington palm as being caused by the P. drechsleri-cryptogea complex. We discuss P. drechsleri in the following paragraphs and then discuss current progress in resolving the taxonomic problem.
1. Sporangia
Sporangia vary in shape from broadly obpyriform to elongated obpyriform, ovoid to elongate (occasionally tapering at the base) (Klisiewicz 1977). They are nonpapillate and persistent on the stalk. Sizes are variable (Table 23.2), in general 36 to 70 µm long x 26 to 40 µm wide, average 39 x 24 µm, and they proliferate internally or externally. Data in Tables 4.2 and 4.3 are from Tucker (1931). (See Figures 4.9A, 4.13, and 4.14 in Chapter 4.) Sporangiophores are usually narrow but sometimes widen just below the sporangium and form in a sympodium (Figure 1).
2. Chlamydospores
Chlamydospores are reported only in some isolates (Frezzi 1950; Katsura 1971; Cother and Griffin 1973b, 1974) and measure 4.2 to 11.0 µm in diameter, average 7.9 µm.
3. Sex Organs
P. drechsleri is usually heterothallic, forming oospores only when paired with A1 or A2 mating types; however, Tucker (1931), Tompkins et al. (1936a), and Klisiewicz (1977) report that some isolates produce oospores in single culture. Although P. drechsleri f. sp. cajani was found to be heterothallic when paired with A2 isolates of other species, oospores formed in single culture when incubated at 30oC (Kannaiyan et al. 1980). Singh and Chauhan (1988) reported that oospores formed in single culture when pigeon pea leaves were inoculated with zoospores. Antheridia are amphigynous; oogonia have tapered bases and are 22 to 53 µm in diameter, average 33 µm; and oospores are plerotic and 17 to 50 µm in diameter, average 26 µm. See Table 2 for references on oospore and sporangium sizes.
4. Growth Temperatures
Minimum temperature for growth is 5oC, optimum 28 to 31oC, and maximum 35 to 37oC (Tucker 1931). Many isolates produce optimal growth at 30oC with some growth at 35oC (Tucker 1931; Erwin 1952; Tompkins et al. 1936a; Mills et al. 1991) (Table 1). Colonies form in rosaceous, stellate, or cottony patterns; however, these patterns are not consistent for all isolates (Figures 4.2 and 4.3 in Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996).
5. Distinguishing Characteristics
Hyphal swellings are round or angular, form in chains or clusters, and are similar to those of P. cryptogea; see Figure 4.4A in Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996). Phytophthora root rot of safflower was originally reported to be caused by P. drechsleri, as identified by Tucker (see Erwin 1950). The shape of the base of the sporangium was used as a criterion to differentiate P. cryptogea from P. drechsleri (Klisiewicz 1977) (Figure 2). Kliesewicz (1977) described cultures from safflower that grew at 35oC, some of which he designated P. drechsleri because the sporangia were elongated to ellipsoid and the bases were sloping and others P. cryptogea because sporangia had rounded bases (Figure 2). Although Klisiewicz (1977) did not present data for the population of sloping-base or rounded-base types, he stated that the majority of sporangia of P. drechsleri had sloping bases and sporangia of P. cryptogea had rounded bases. Mircetich and Matheron (1976) also illustrate sporangia with sloping bases to be typical of P. drechsleri from cherry trees (their Figure 2H). Isolates from bean produced sporangia with both tapered and rounded bases and were designated P. cryptogea (Flowers et al. 1973). Ho and Jong (1986) compared 15 isolates previously identified as P. cryptogea and 14 isolates previously identified as P. drechsleri from a wide range of sources, but morphological characters, including rounded or tapered sporangial bases, were inconsistent. Likewise, Mills et al. (1991) found that this character was too inconsistent to be reliable. Ho and Jong (1986) considered merging the two species but refrained pending further study on more isolates. Their evidence for merging the two species was convincing until a study of a larger number of isolates of both species by isozyme and mitochondrial DNA restriction fragment length polymorphism (mtDNA RFLP) patterns showed that 10 groups could be differentiated within the P. cryptogea-P. drechsleri complex. Differences between some of the groups were as great as those among certain other valid Phytophthora species (Mills et al. 1991). Both reports, as well as a subsequent 1991 report by Ho and Jong in which P. cryptogea was redescribed without being merged with P. drechsleri, are reviewed below. Ho and Jong (1986) noted that colony morphology of P. drechsleri and P. cryptogea isolates was similar on cornmeal agar; sporangia of most isolates were obpyriform, ovoid, or elongate, with either tapered or rounded bases. In seven of 19 cultures of P. cryptogea and in seven of 14 isolates of P. drechsleri, sporangia with both rounded and tapered bases formed; however, tapered bases were more common on P. drechsleri sporangia. Most isolates were heterothallic, and oogonia were spherical to subspherical without tapered bases. Smooth, globose oospores were mostly plerotic. Chlamydospores were not noted in culture. All isolates produced pigmentation on a casein hydrolysate medium (Timmer et al. 1970), tolerated malachite green (dilution of 1:18 x 106), and utilized inorganic nitrates in a synthetic medium. When cultures were incubated at 35oC, all but five of 14 isolates previously designated P. drechsleri grew. At 35oC, one of 19 isolates previously designated P. cryptogea grew well, three showed only slight growth, and the remainder did not grow. Broad hyphae (>8 µm in diameter) were cited by Newhook et al. (1978) as characteristic of P. cryptogea, whereas narrower hyphae (<8 µm in diameter) are characteristic of P. drechsleri, but Ho and Jong (1986) did not note any consistent difference in hyphal breadth between the two species. Size of sporangia for P. cryptogea (small, 37 to 40 µm long) and P. drechsleri (large, 36 to 50 µm long) was cited by Waterhouse (1963) to be a differential character; however, Ho and Jong (1986) found that the sizes of sporangia of P. drechsleri isolates was 52 
10 µm versus 49 8 µm for P. cryptogea. The length-breadth ratio was 1.7 0.2 for the P. cryptogea isolates versus 1.9 0.3 µm for the P. drechsleri isolates (Ho and Jong 1986), hardly enough to differentiate species. In 1991, Ho and Jong examined more isolates of P. drechsleri and P. cryptogea, concluded that these species should not be merged, redescribed P. cryptogea to include those isolates intermediate between P. cryptogea and P. drechsleri that did not grow at 35oC, but retained P. drechsleri to include those isolates that grew well at 35oC. P. cajani (Amin et al. 1978), P. erythroseptica var. drechsleri (Sarejanni 1936), P. melonis (Katsura 1976), and P. sinensis (Yu and Zhuang 1982) were included as conspecific with P. drechsleri (also by Ho in 1981). Their use of growth at 35oC as a differential character actually revives the criterion first established by Tucker (1931) to differentiate P. cryptogea from P. drechsleri. Mills et al. (1991) concluded from their molecular studies, in which isozyme and mtDNA RFLP patterns (see Chapters 3 and 4 in Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996) for a discussion of the methods and rationale [Spielman 1991]) were used to categorize isolates, that the P. cryptogea-P. drechsleri complex was too diverse to warrant a merger of the two species and postulated that there were probably seven genetic species within this group. After analysis via isozymes and mtDNA RFLP technology of 129 isolates previously identified as P. drechsleri, P. cryptogea, P. melonis, and P. drechsleri f. sp. cajani and selected from a wide range of geographical areas and crops, Mills et al. (1991) grouped the cultures according to their genetic relatedness. A phenogram demonstrating the diversity in mtDNA RFLP patterns is shown in Figure 4.33 (F?ster and Coffey 1991 Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996); summarized from Mills et al. 1991). Group A was homogeneous and included the P. drechsleri culture from potato originally described by Tucker (1931), three isolates from rotted roots of safflower that had originally been named P. drechsleri by Tucker (Erwin 1952) but had been renamed P. cryptogea on the basis of the production of sporangia with tapered bases (Klisiewicz 1977), isolates causing root rot of sugar beet (described by Tompkins et al. [1936a]) (Plate 23.1), and an isolate causing root rot of sweet pepper. All of these isolates grew well at 35oC, and the nonpapillate, ovoid, ellipsoid, obpyriform sporangia were typical of the criteria for P. drechsleri formulated by Tucker (1931) and continued by Waterhouse (1963), Newhook et al. (1978) in the revisions of her key, and Stamps et al. (1990); however, the criterion of rounded or tapered sporangial bases was not consistent. Many isolates produced both rounded- and tapered-base sporangia. They noted a few isolates in groups G and H that had predominately tapered sporangial bases and some isolates in groups J and K that produced mainly sporangia with tapered bases (see Figure 2 and 4.14 in Erwin and Ribeiro (1996, Phytophthora Diseases Worldwide). Isolates of P. drechsleri f. sp. cajani (group G) produced sporangia with both rounded and tapered bases (Figure 4.13 Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996). A molecularly different group B contained what Mills et al. (1991) considered to be authentic cultures of P. cryptogea that did not grow at 35oC but were similar morphologically to both P. drechsleri and P. cryptogea. The three cultures previously designated P. erythroseptica that occurred in group B were not explainable but could have resulted from labeling errors or to misidentification by those who donated the cultures. Group F was distinct and included a number of isolates from melons (Cucumis sativa), some of which had been previously identified as P. melonis. These isolates were similar morphologically and by growth at 35oC to group A and were pathogenic to cucumber but not to tomato. Mills et al. (1991) recommended that these not be placed in P. drechsleri but that an amended description of P. melonis should be considered. To date, an amended description has not been made. This description is needed because the sporangia of P. melonis were originally described as semipapillate by Katsura (1976); however, Ho (1986) observed that P. melonis and P. sinensis produced nonpapillate sporangia. He considered them to be synonymous with P. drechsleri. Group G of Mills et al. (1991) was comprised of isolates from pigeon pea (Cajanus cajan) originally designated P. cajani (Amin et al. 1978) and reassigned to P. drechsleri f. sp. cajani (Kannaiyan et al. 1980). Mills et al. (1991) recommended that these host-specific isolates, which grew at 35oC, be returned to P. cajani (see Chapter 12 Phytophthora Diseases Worldwide (Erwin and Ribeiro 1996).). We agree that this recommendation should be considered, but more definitive data are needed. Amin et al. (1978) described the pigeon pea pathogen as homothallic, but this was not confirmed by Kannaiyan et al. (1980). The cause of Phytophthora blight is described here under P. drechsleri f. sp. cajani. Group C, which did not grow at 35oC, consisted of isolates from woody hosts that had been identified in different reports as P. cryptogea. They were unique in that tests with the isozyme G-6-pi (glucose-6-phosphate isomerase), known to be a dimeric enzyme (Tooley et al. 1985), showed five-banded patterns. These isolates with heterozygous phenotypes, in which three or four alleles were involved, appeared to be polyploids. Group D, which did not grow at 35oC, included only the host-specific P. cryptogea f. sp. begoniae isolates from Begonia elatior in Germany (Kr?er 1981b). This group should be restudied and possibly described as a new species, if warranted. Group E isolates, which did not grow at 35oC, were from grape, pine, and apple and were isozymically similar to group D. Group H contained seven isolates, all of which grew at 35oC, from deciduous fruit trees and vines (walnut, apple, cherry, and kiwi) in California and Arizona that had been previously identified as P. drechsleri. Groups J and K, which did not grow at 35oC, were isozymically and genetically unrelated to each other and contained isolates previously identified as P. cryptogea from peach, cherry, apple, fir, and pine trees. Brasier and Hansen (1992) suggest that isolates in group J may represent P. gonapodyides. Another group was isozymically too diverse to categorize and consisted of seven isolates previously designated P. drechsleri, nine P. cryptogea, and two unidentified. Data on growth at 35oC was not given for these. The morphology of sporangia or oogonia was insufficient to separate species as similar as P. drechsleri and P. cryptogea, but Mills et al. (1991) concluded that these two species should not be merged because many of the groups within them were unrelated in isozyme and mtDNA RFLP patterns. An interesting sidelight was the inclusion in group A of an isolate that had been received with the identity of P. erythroseptica. Upon reexamination of the morphology, it was found to be heterothallic and grew well at 35oC, traits that were characteristic of P. drechsleri but not P. erythroseptica. F?ster and Coffey (1991) referred to the seven groups as molecular species instead of biological species. Many of the groups differed genetically as much as the known species P. cinnamomi, P. lateralis, and P. erythroseptica, which were included as \'out species\' in the study. After reviewing the careful studies by Ho and Jong (1986, 1991) and Mills et al. (1991), we are left with the practical dilemma that previous methods of separating P. cryptogea from P. drechsleri are inadequate and that more undescribed species probably exist within the complex. Brasier and Hansen (1992) postulate that the data of Mills et al. (1991) suggest that morphologically similar species may have evolved via geographical isolation and climatic and host specialization, and that their distributions are now overlapping through introductions. This overlapping may also have led to hybridization and the emergence of new taxa. Sexual crosses between some of the groups might help test their biological species status and whether or not they need to be formally recognized. These ideas are speculative. One problem in interpretation of the data to distinguish P. cryptogea from P. drechsleri lies in the lack of a firm establishment of what qualities characterize the species that Tucker (1931) established as P. drechsleri. Apparently, the species he described was represented by only one isolate. The grouping from a sample of 129 isolates tested by Mills et al. (1991) shows that group A includes what could be considered typical P. drechsleri sensu Tucker (1931), because all of these isolates grew well at 35oC. From a practical standpoint, growth at 35oC appears to be a consistent feature in the groups associated with P. drechsleri. Groups F, G, and H, however, all of which were previously considered to be P. drechsleri by other references, also grow at 35oC, and each of these groups is as genetically different from group A (probably typical P. drechsleri) and from each other as they are from many other species. Ho and Jong (1986) showed that a few of the isolates previously designated P. cryptogea grew at 35oC and a few of those designated P. drechsleri did not grow at 35oC. The problem here is the unclear basis on which isolates were designated as one or the other species in the Mills et al. (1991) or Ho and Jong (1986) reports. For instance, within group A (Mills et al. 1991), three isolates listed as P. cryptogea from safflower had been identified as P. drechsleri for many years before being reassigned to P. cryptogea by Duniway (1976) and Klisiewicz (1977) on the basis of sporangial morphology. Another example of the lack of a firm baseline for comparison of P. cryptogea and P. drechsleri is in the report by Bumbieris (1974), who compared mycelial width, sporangial shapes, growth in a medium amended with malachite green (0.1 ppm), and growth at 35oC for five isolates designated P. drechsleri and seven isolates designated P. cryptogea; however, only the isolate considered to be an authentic P. drechsleri isolate (I.M.I. 40499) obtained from the Commonwealth Mycological Institute (CMI) in the United Kingdom grew at 35oC. All other isolates of either species did not grow at 35oC and were indistinguishable by any of the other criteria measured. Although this study has long been quoted to justify merging the two species, if the isolate I.M.I. 40499 from CMI were the only valid P. drechsleri isolate and if growth at 35oC were a valid criterion, this study does not support merging the two species because all of the isolates with one exception may have been P. cryptogea. Previous identification of an isolate does not necessarily make it a reliable item for comparison. Because it is now clear that sporangial shape or mycelial width morphology cannot separate the two species (Figures 4.13 and 4.14 show both rounded- and tapered-base sporangia produced in one culture of P. drechsleri), the only character left is growth at 35oC. Admittedly, growth at a high temperature is not a morphological character, but it is the only character until a better one is found. The mtDNA RFLP and isozyme data tell us that there may be differences that have not been seen and should be looked for. In the absence of more definitive data, the amended description by Ho and Jong (1991), which cites growth at 35oC, has to be accepted. The molecular-species concept of Mills et al. (1991) should be considered, but it is not yet well enough defined to use in a practical way. The amended description of P. drechsleri by Ho and Jong (1991) follows (reproduced by permission of Mycotaxon Ltd., Ithaca, New York, courtesy of Ho and Jong 1991): Phytophthora drechsleri Tucker emend., 1931. = P. cajani Amin, Baldev et Williams, Mycologia 70:174, 1978. = P. drechsleri var. cajani Pal, Grewal et Sarboy, Indian Phytopathol. 23:585, 1970. = P. erythroseptica var. drechsleri Sarejanni, Ann. Inst. Phytopathol. Benaki 2:48, 1936. = P. melonis Katsura, Trans. Mycol. Soc. Japan 17:238, 1976. = P. sinensis Yu et Zhuang, Mycotaxon 14:183, 1982.
[[PAPER:2342|1]]
Nomenclature information was provided by the the Systematic Botany and Mycology Laboratory in USDA-ARS.
Isolate list