Natural Occurrence of Phytophthora infestans Causing Late Blight on Woody Nightshade (Solanum dulcamara) in New York.

Woody nightshade (Solanum dulcamara) is a common hedgerow herbaceous perennial in the United States, one of only three native Solanum spp. S. dulcamara is a known host of Phytophthora infestans (3), but infection is rarely reported. There is a U.S. record from Maryland (2); in 1947, Peterson (4) stated that this species had never been found blighted in its natural habitat, although in 1960 it was listed as a host of P. infestans in New York (1). The A2 mating type has not been reported on this host. On 2 July, 2009, leaf lesions similar to those of P. infestans on potato were found on wild S. dulcamara at Riverhead, NY. The plant was growing in a home garden within 10 m of potato and tomato plants infected with P. infestans. When two infected leaves of S. dulcamara were incubated for 24 h under high humidity, a pathogen growth developed around the lesion margins that was characterized by hyaline mycelium bearing lemon-shaped sporangia that released motile zoospores after chilling in water, which is consistent with P. infestans. The caducous and limoniform to ovoid sporangia were 39 to 50 µm (average 45 µm) × 26 to 28 µm (average 27 µm) with a length/breadth ratio of 1.66. No oospores were observed. Three isolates were obtained from this plant during July 2009. Growth on rye agar was indistinguishable from that of local tomato isolates of P. infestans. Detached leaflets of S. dulcamara and S. tuberosum, inoculated with the woody nightshade isolates and kept in a humid chamber, became infected and developed profuse sporulation within 5 days. The pathogen isolated was confirmed as P. infestans by morphological, biochemical, and molecular characteristics. Inoculations of attached leaves of potted S. dulcamara plants resulted in necrotic lesions with many sporangia; sporulation also developed on inoculated, attached, and detached tomato leaves. P. infestans was reisolated and identity confirmed as before. The three isolates were A2 mating type, metalaxyl-resistant, mitochondrial haplotype Ia. All were glucose-6-phosphate isomerase 100/122 and peptidase 100/100, as confirmed with single-spore isolates. RG57 fingerprint analysis confirmed that isolates from woody nightshade, tomato, and potato obtained from the same and nearby sites were identical. Although P. infestans in the United States belongs to the new population, which may infect a wider host range than the old US-1 clonal lineage, S. dulcamara infections have only been found when late blight is already widespread in neighboring fields and there is no evidence to suggest that woody nightshade acts as an overwintering host in the United States. References: (1) Anonymous. Index of Plant Diseases in the United States. Page 456 in: Agric. Handb. No. 165, 1960 (2) C. Cox. Phytopathology 38:575, 1948. (3) D. C. Erwin and O. K. Ribeiro. Page 190 in: Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (4) L. C. Peterson. Am. Potato J. 24:188, 1947.

Formation and germination of Phytophthora infestans oospores in different regions of Russia.

On occurrence of oospores (sexual stage), 88 Phytophthora infestans populations were investigated. The populations were collected in 14 regions of Russia. In total, 3677 samples have been checked: 2888--from potato leaflets (55 populations), 344--from tomato leaflets (16 populations), 445--from tomato fruits (17 populations). The oospores have been detected both in potato and tomato leaflets and in fruits with different occurrence--4.4%, 11.9%, and 28.8%. Occurrence of oospores per sample was also maximal in tomato fruits. After overwintering a majority of oospores (50-90%) kept their viability. Sometimes, loss of viability of some oospores was caused by the external negative influence. About 25% of oospores formed at crossing between current Russian P. infestans strains were capable to germinate. Treatment with negative temperature (-12 degrees C) or Trichoderma suspension increased percentage of germinated oospores. In some field trials, the oospores caused affection of quite many of potato plants. However, in other trials only particular plants or no plants were affected. Tomato germlings were affected only under unfavourable conditions.

First Report of the A2 Mating Type of Phytophthora infestans on Tomato Crops in Taiwan, Republic of China.

In a study of the Phytophthora infestans population in Taiwan, samples with symptoms typical of late blight were collected from field crops in an important potato- (Solanum tuberosum) and tomato-(Lycopersicon esculentum) production area in the central highlands region. Isolates were obtained by surface disinfecting leaf sections and plating them onto antibiotic-amended rye A agar (1). After subculturing, the pathogen was confirmed as P. infestans on the basis of morphological characters (2). Mating type was determined by co-inoculating unamended rye agar plates with mycelial plugs of the test isolate and a reference P. infestans isolate of either the A1 or A2 mating type (four plates per test isolate, two with different A1, and two with different A2 reference isolates). After incubation (15°C darkness, 7 to 14 days), plates were examined microscopically for the presence of oospores where the colonies interacted. In 2004, one isolate of 200 tested, and in 2006, one isolate of 102 tested, produced oospores only with A1 reference isolates and were concluded to be A2 mating type. In vitro testing showed the two A2 isolates were metalaxyl-resistant (ED50 values >100 mg of metalaxyl per liter on rye grain agar), which is typical of recent P. infestans isolates from potato and tomato in this area (2). Twenty-one single-sporangial isolates from each of the two A2 strains were tested for mating type against two different A1 isolates of P. infestans and confirmed as A2. These isolates were characterized using the techniques described by Deahl et al. (1) and had the allozyme genotype 100/100/111, 100/100 at the loci coding for glucose-6-phosphate isomerase and peptidase, respectively, and were mitochondrial haplotype IIb. This multi-locus genotype is characteristic of recent P. infestans isolates from tomato and potato in Taiwan, but all previous such isolates were A1 mating type and attributed to the US-11 clonal lineage (1). When evaluated on differential hosts, both A2 isolates were tomato race PH-1 and complex potato race R 0,1,2,3,4,7,9,11. RG57 fingerprinting showed that the A2 isolates had fingerprints identical to each other and to A1 P. infestans isolates of the US-11 clonal lineage from tomato in Taiwan (101 011 100 100 110 101 011 001 1). Koch's postulates were completed and the two A2 isolates were found to be highly aggressive on cultivars of potato and tomato. To our knowledge, this is the first report of A2 mating type strains of P. infestans in the field in Taiwan, but currently, their incidence is very low (<1%). One crop from which an A2 isolate was obtained also yielded an A1 isolate, while A1 isolates were obtained from crops in the vicinity of the other. The concurrent presence of the two mating types of P. infestans poses a risk of sexual reproduction and oospore formation in tomato or potato in Taiwan. References: (1) K. L. Deahl et al. Pest Manag. Sci. 58:951, 2002. (2). D. C. Erwin and O. K. Ribeiro, Page 346 in: Phytophthora Diseases Worldwide. The American Phytopathological Society. St. Paul, MN, 1996.

Occurrence of Late Blight Caused by Phytophthora infestans on Potato and Tomato in Alaska.

Phytophthora infestans, causal agent of late blight, was included in a list of plant pathogens found in Alaska in 1934 (1). No notes of symptoms, extent of disease, or dates were recorded. The only reference to the location was given as Wrangell, a town in southeast Alaska with subsistence gardening. Neither P. infestans nor late blight was noted again in the state for another 59 years. Late blight first appeared in Alaska's major potato-growing region in south-central Alaska's Matanuska Valley in 1995. Subsequent outbreaks have been sporadic, occurring only in 1998, 2005, and 2006. Each of these outbreaks was identified from rapidly enlarging brown foliar lesions with branched sporangiophores and lemon-shaped sporangia (~25 × 30 µm). The 1995 and 1998 potato late blight outbreaks in Alaska were not sampled extensively nor have they previously been formally reported. We recovered single isolates of P. infestans from symptomatic potato foliage in the 1995 and 1998 outbreaks. In 2005, symptomatic foliage was collected from individual potato plants in 10 commercial fields and from tomato plants in greenhouses at two locations. Sporulating stem and leaf tissue were used to inoculate semiselective rye medium and 147 isolates from potato and six from tomato were recovered. The isolates from the 1995, 1998, and 2005 outbreaks were analyzed to determine genotype at the allozyme loci GPI and PEP (3), mitochondrial haplotype (4), mating type, and metalaxyl sensitivity (2). The 1998 and 2005 outbreaks were similar because both were caused by the relatively aggressive US-11 allozyme genotype and had significant economic impact for commercial potato growers. All 153 isolates from potato and tomato in 2005 displayed the same allozyme pattern as the US-11 genotype, possessed the IIB mitochondrial haplotype, and were mating type A1. Of the 16 isolates tested, all were determined to be metalaxyl resistant because isolates grown on 5 and 100 µg/ml metalaxyl exhibited growth greater than 40% of the unamended control. The 1995 outbreak was caused by the relatively rare US-7 genotype and started so late during the season that economic impact was minimal. Similarly, the 2006 outbreak was noted from only one commercial potato field at the time of harvest in September 2006. However, the genotype of the 2006 isolate has not been determined because the patch was destroyed before adequate samples could be collected. Because the disease occurs so sporadically in Alaska, fungicides are not routinely in use, but it is unlikely that the pathogen has persisted locally between outbreaks. The source of P. infestans is unknown for each of the occurrences in Alaska. However, possible routes include seed potatoes for home gardens or commercial farms, tomato transplants, and retail vegetables shipped to Alaska from out of state. While potato is Alaska's main vegetable crop, there are less than 405 ha (1,000 acres) of potatoes planted in the state, with the majority planted in the Matanuska Valley. To our knowledge, this is the first formal report of P. infestans on both tomato and commercial potato in Alaska. References: (1) E. K. Cash. Plant Dis. Rep. 20:121, 1936. (2) D. E. L. Cooke et al. Plant Pathol. 52:181, 2003. (3) S. B. Goodwin et al. Plant Dis. 79:1181, 1995. (4) G. W. Griffith and D. S. Shaw. Appl. Environ. Microbiol. 64:4007, 1998.

Gene duplication event in family 12 glycosyl hydrolase from Phytophthora spp.

A total of 18 paralogs of xyloglucan-specific endoglucanases (EGLs) from the glycosyl hydrolase family 12 were identified and characterized in Phytophthora sojae and Phytophthora ramorum. These genes encode predicted extracellular enzymes, with sizes ranging from 189 to 435 amino acid residues, that would be capable of hydrolyzing the xyloglucan component of the host cell wall. In two cases, four and six functional copies of these genes were found in tight succession within a region of 5 and 18 kb, respectively. The overall gene copy number and relative organization appeared well conserved between P. sojae and P. ramorum, with apparent synteny in this region of their respective genomes. Phylogenetic analyses of Phytophthora endoglucanases of family 12 and other known members of EGL 12, revealed a close relatedness with a fairly conserved gene sub-family containing, among others, sequences from the fungi Emericella desertorum and Aspergillus aculeatus. This is the first report of family 12 EGLs present in plant pathogenic eukaryotes.

Late Blight Caused by Phytophthora infestans on Solanum sarrachoides in Northeastern Maine.

The area bordering three 110-ha (270-acre) fields of blighted potatoes (Solanum tuberosum L.) in three northeastern Maine locations was surveyed during the summer of 2004 for the occurrence of late blight on cultivated and noncultivated host plants. Special attention was directed to solanaceous weed species. Hundreds of Solanum sarrachoides Sendt. ex. Mart. (hairy nightshade) plants with numerous leaf lesions and moderate defoliation were seen. The frequency of blighted hairy nightshade approximated the frequency of late blight in the adjoining potato fields. Lesions typically contained extensive, white, superficial mycelia colonizing the abaxial and adaxial leaf surfaces. Samples placed in a moist chamber produced lemon-shaped sporangia. On the basis of morphological characteristics, the pathogen was tentatively identified as Phytophthora infestans (Mont.) de Bary. Isolates were obtained by surface-disinfecting leaf sections collected from two locations for 2 to 3 min in 0.5% NaOCl and placing the sections on rye grain medium amended with antibiotics (100 ppm each of penicillin G, pimaricin, and polymyxin). P. infestans was confirmed after reisolating onto rye-lima bean medium. Pathogenicity was tested on detached potato, tomato, and hairy nightshade leaves; the undersides of all leaflets from replicate plants were inoculated with droplets of swimming zoospores (≥500 zoospores per droplet), the leaves were kept at 17°C and 100% humidity, and the extent of sporulation was evaluated after 4, 6, and 7 days. With eight isolates obtained from S. sarrachoides, Koch's postulates were completed on potato and hairy nightshade. Radial growth responses of these strains on rye grain agar amended with 1, 10, or 100 µg per ml of metalaxyl (Ridomil 2E) yielded 50% effective dose values greater than 100 µg per ml, since percentage growth at the highest fungicide concentration exceeded 50% of the no metalaxyl control. These resistance levels are typical of the metalaxyl-insensitive strains of P. infestans isolated from potatoes in this area in recent years, which were previously found to correlate with metalaxyl resistance in bioassays using potato tissues (1). Eight single-sporangial isolates were homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/111/122, Pep 100/100). All eight were A2-mating type and mitochondrial haplotype Ia, characteristics common to the US-8 clonal lineage of P. infestans from potato (2), which may infect a wider host range than the old US-1 clonal lineage. When evaluated on differential hosts, three isolates were tomato race PH-1 and complex potato race R 0,1,2,3,4,9,11. DNA fingerprint analysis with probe RG57 further established that the eight hairy nightshade isolates were identical to each other and to local P. infestans isolates from potato. To our knowledge, this is the first report of infection of S. sarrachoides by P. infestans in Maine. The pathogen was previously isolated from this host during field surveys in southern California in the 1980s in connection with late blight of tomato (4). Hairy nightshade has been shown to be a host for US-1, US-8, and US-11 isolates of P. infestans in a laboratory setting (3). The epidemiological significance of S. sarrachoides as an alternative or overwintering host of P. infestans is currently being assessed. References: (1) K. L. Deahl et al. Am. Potato J. 70:779, 1993. (2) S. B. Goodwin et al. Phytopathology 88:939, 1998. (3) H. W. Platt. Can. J. Plant Pathol. 21:301, 1999. (4) V. G. Vartanian and R. M. Endo Plant Dis. 69:516, 1985.

Natural Occurrence of Phytophthora infestans on Black Nightshade (Solanum nigrum) in Wales.

There is only one published record of natural infection of black nightshade (Solanum nigrum L.) by Phytophthora infestans (Mont.) de Bary in England (3) and none from Wales. In August 2001, brown, necrotic leaf lesions with pale green margins were found on black nightshade weeds in a potato trial naturally infected with P. infestans at Henfaes Research Centre, University of Wales, Bangor. Although the plants were low growing with large, succulent leaves 4 to 5 cm long instead of having a more erect habit and smaller leaves, their identity was confirmed as S. nigrum; their atypical appearance may relate to the known phenotypic plasticity of this species (4). Infected leaflets incubated in moist chambers produced sporangia typical of P. infestans, and zoospores were released after chilling in water. Five isolates obtained from leaf fragments had growth on rye agar that was indistinguishable from that of P. infestans from potato. Detached leaflets of S. nigrum and S. tuberosum cv. Green Mountain inoculated with the S. nigrum isolates developed sporulating lesions under high humidity in 7 to 10 days; uninoculated controls remained symptomless. Inoculation of attached leaves of 10 potted S. nigrum plants resulted in seven plants developing necrotic lesions with a few sporangia 10 to 14 days later; sporulation developed mainly on lower leaves of plants that were older or had senesced. The remaining plants developed necrotic lesions with no sporulation, and P. infestans was reisolated from sporulating and nonsporulating lesions. All isolates were A1 mating type, metalaxyl-sensitive, and mitochondrial haplotype IIa, which are characteristics found commonly in isolates of P. infestans from potato in Wales (1). Single-sporangial isolates from each isolate were homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100). RG57 fingerprint analysis further established that all five black nightshade isolates were identical to each other and to some local P. infestans isolates from potato. P. infestans in Wales belongs to the new population (1), which may infect a wider host-range than the old US-1 clonal lineage. However, infected black nightshade was only found after late blight was widespread in potato fields. In subsequent years at the same site, weeds of S. nigrum have remained noninfected despite high levels of late blight pressure on adjacent potato plots. There is no evidence to suggest that this species acts as an overwintering host in Wales since it is an annual and lacks frost resistance. Field infection of S. nigrum by P. infestans has recently been reported in the Netherlands (2). Our observations confirm the potential of P. infestans to infect another solanaceous plant species. Alternative hosts may interfere with current disease control strategies because infected weeds would escape fungicide application and could serve as reservoirs of inoculum throughout the growing season. References: (1) J. P. Day and R. C. Shattock. Eur. J. Plant Pathol. 103:379, 1997. (2) W. G. Flier et al. Plant Pathol. 52:595, 2003. (3) J. M. Hirst and O. J. Steadman. Ann. Appl. Biol. 48:489, 1960. (4) B. S. Rogers and A. G. Ogg Jr. Page 30 in: Biology of Weeds of the Solanum Nigrum Complex (Solanum Section Solanum) in North America. USDA Publication ARM-W-23, 1981.

Occurrence of Leaf Blight on Petunia Caused by Phytophthora infestans in Maryland.

In April 2001, late blight was detected on petunia (Petunia × hybrida Hort.) in a glasshouse in central Maryland. Symptomatic plants had extensive light gray, irregularly shaped, slightly sunken lesions up to 3 cm long on upper leaves and branches. Stems were distorted or dead above points where lesions had coalesced to cause stem girdling. Disease initially occurred as scattered foci but rapidly spread through the crop causing significant loss. The disease reappeared in the same and adjacent greenhouses in 2002. Phytophthora infestans Mont deBary was observed sporulating on leaves and stems of plants in both years. Isolates PE-01, PE-02, and PE-03 were isolated on rye medium from symptomatic stem and leaf tissue (1). To confirm pathogenicity, leaves and stems of 6-wk-old petunia (cv. White Madness) were sprayed with water or with aqueous suspensions of 105 sporangia per ml. Plants were placed in plastic bags in a growth chamber at 20 to 22°C. Bags were removed after 3 days. Five to seven days after spraying, necrotic leaf lesions similar to those on naturally infected plants developed on inoculated plants only. Lesions occurred at leaf margins or near midribs and were 1 to 6 mm, irregular in shape, slightly sunken, grayish, and accompanied by white, fluffy mycelial and sporangial growth. P. infestans was reisolated onto rye agar from lesions from all inoculated plants, completing Koch's postulates. Lesions contained abundant, nonseptate sporangia on obvious sporangiophore structures whose morphology matched that of P. infestans (2). Sporangia from these lesions were 39 to 50 µm (average 45 µm) long × 26 to 28 µm (average 27 µm) wide with a length/breadth ratio of 1.66. Sporangia were caducous and limoniform to ovoid in shape. Oospores were not observed. The fungus grew more slowly on potato dextrose agar (PDA) than on rye medium at 18°C (2.7 versus 9.8 mm per day) and 25°C (1.6 versus 3.0 mm per day). All three isolates mated with an A1 type isolate of P. infestans forming oogonia (36.14 µm in diameter), amphigynous antheridia (22.39 µm × 17.00 µm wide), and aplerotic oospores (16.91 µm in diameter), indicating that the petunia isolates were the A2 mating type. These characteristics were consistent with a heterothallic species such as P. infestans (3). Tomato and potato plants were susceptible to PE-01. The three petunia isolates were US-8 multilocus genotype, resistant to Ridomil and mitochondrial haplotype Ia, characteristics typical of P. infestans from potato in North America. All isolates were monomorphic and homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100; Pep 100/100). Restriction fragment length polymorphism (RG-57) analysis confirmed the close similarity of the petunia and potato isolates. Greenhouse growers who cultivate more than one Solanaceous species should be aware that petunia transplants may have incipient Phytophthora spp. infections serving as reservoirs of inoculum, and that some strains are resistant to metalaxyl-based fungicides. P. infestans has been reported to cause leaf blight on petunia in the northeastern United States (2,4). To our knowledge, this is the first report of natural infection of P. infestans on petunia in Maryland. References: (1) C. E. Caten and J. L. Jinks. Can. J. Bot. 46:329, 1968. (2) M. Daughtrey and M. Becktell. Grower Talk Mag. 66:90, 2002. (3) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society St. Paul, MN, 1996. (4) M. C. Rathbone et al. (Abstr.) Phytopathology 92 (Suppl.):S145, 2003.

Differential neurosensory responses of adult Colorado potato beetle, Leptinotarsa decemlineata, to glycoalkaloids.

Neurons from chemosensory hairs on the galeae of adult Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), were investigated for responses to glycoalkaloids of the family Solanaceae. While solanine and tomatine elicited irregular firing by multiple neurons and bursting activity at 1 mM concentration in most sensory hairs, stimulation with leptine I resulted in consistently high-frequency, slowly adapting responses with a dose-dependent effect between 0.03 and 0.3 mM concentrations. Responses to a mixture of solanine and leptine I suggested possible modification of the leptine I response by other glycoalkaloids, resulting in reduced neural activity relative to leptine I alone. These results establish a method for specifically evaluating leptine I and other glycoalkaloids for effects on feeding behavior of CPB and provide a sensory component for incorporating deterrent chemistry into biorational control methods for the CPB.

Partial preparative purification of leptine I from foliage of the wild potato, Solanum chacoense (Bitt.).

Leptine I, a glycoalkaloid only known to occur in the foliage of the wild potato species Solanum chacoense (Bitt.), is a potent feeding deterrent to the economically serious insect pest, the Colorado potato beetle (Leptinotarsa decemlineata Say). In order to demonstrate, systematically, the effectiveness of leptine I, incorporation into synthetic beetle diet trials is necessary. We describe a preparative procedure for the partial purification of leptine I by a series of steps, starting with a solid-phase C18 extraction, followed by sequential silica gel chromatography, and finally reversed-phase preparative HPLC. This preparation yields a white powder, containing leptine I as the sole glycoalkaloid, with an overall purity of greater than 65%, and is entirely suitable for incorporation into synthetic diets.

Characterization of Phytophthora infestans Populations in Western Washington.

The first detection in the United States of isolates of Phytophthora infestans having metalaxyl insensitivity and complex pathotypes occurred in western Washington during the early 1990s. To determine the genetic structure of the current population in western Washington, a total of 115 isolates of P. infestans were obtained during 1996 from infected tubers or foliage of potato, tomato, nightshade, and bittersweet throughout the region. An additional 45 isolates were collected from a single field. Based on mating type, metalaxyl-insensitivity, and molecular markers (allozymes of glucose-6-phosphate isomerase, peptidase, and RG57 DNA fingerprint), all of the isolates were A1 mating type and had the US-11 multilocus genotype. Analyses of an additional 120 isolates collected during 1997 from potato, tomato, and nightshade were performed. As in 1996, US-11 was the predominant genotype detected on all three hosts. However, three additional A2 mating type genotypes were also detected: US-7, US-8, and US-14. These three genotypes represent the first A2 mating type isolates detected in western Washington. Most of a subset of 60 isolates infected 4 to 7 of the 10 potato differentials tested. This included 90% of the isolates collected in 1996 (all US-11), plus 72% of the US-11 and 100% of the US-8 and US-14 isolates collected during 1997. Virulence phenotypes in this region are complex even without the selection pressure of R-genes in the local commercial cultivars. The apparent increase in genetic variation observed in populations of P. infestans in western Washington from 1996 to 1997 most likely occurred by migration rather than by sexual recombination.

Genetic Change Within Populations of Phytophthora infestans in the United States and Canada During 1994 to 1996: Role of Migration and Recombination.

ABSTRACT Dramatic changes occurred within populations of Phytophthora infestans in the United States and Canada from 1994 through 1996. Occurrence of the US-8 genotype, detected rarely during 1992 and 1993, increased rapidly and predominated in most regions during 1994 through 1996. US-7, which infected both potato and tomato and made up almost 50% of the sample during 1993, was detected only rarely among 330 isolates from the United States analyzed during 1994. It was not detected at all in more limited samples from 1996. Thus, ability to infect both potato and tomato apparently did not increase the fitness of this genotype relative to US-8, as predicted previously. US-1, the previously dominant genotype throughout the United States and Canada, made up 8% or less of the samples analyzed during 1994 through 1996. A few additional genotypes were detected, which could indicate the beginnings of sexual reproduction of P. infestans within the United States and Canada. However, clonal reproduction still predominated in all locations sampled; opportunities for sexual reproduction probably were limited, because the A1 and A2 mating types usually were separated geographically. The high sensitivity of the US-1 genotype to the fungicide metalaxyl also could have reduced opportunities for contact between the mating types in fields where this compound was applied. The previous correlation between metalaxyl sensitivity and genotype was confirmed and extended to a new genotype, US-17: all US-1 isolates tested were sensitive; all isolates of the US-7, US-8, and US-17 genotypes tested to date have been resistant. Isolates of P. capsici and P. erythroseptica, two other species often found on tomato and potato, could be easily distinguished from each other and from P. infestans using a simple allozyme assay for the enzyme glucose-6-phosphate isomerase. This technique could be useful for rapid identification of species, in addition to genotype of P. infestans. It generally was not possible to predict which genotypes would be present in a location from 1 year to the next. Long-distance movement of US-8 in seed tubers was documented, and this was probably the primary means for the rapid spread of this genotype from 1993 through 1996.

Bioassays of segregating plants : A strategy for studying chemical defenses.

Solanum chacoense is a wild potato species resistant to the Colorado potato beetle,Leptinotarsa decemlineata. Most genotypes ofS. chacoense synthesize the glycoalkaloids solanine (sol) and chaconine (chac) and are hosts of the beetle. A few rare genotypes have a gene(s) for acetylation of carbon-23 of the steroid aglycone of sol and chac. Laboratory bioassays and replicated field tests of clones differing in the presence or absence of the acetyl moiety showed that acetylation of sol and chac markedly affects the response of both adults and larvae to the foliage. Adult feeding deterrency conferred by acetylated forms of sol and chac (leptines) in leaf-disk preference tests was consistent with the degree of antixenosis measured in the field. Development of larvae on foliage of clones with leptines was also inhibited. The studies support the validity of using laboratory bioassays of plants segregating for levels of a suspected defense compound to determine the role the compound has in defending the plant from attack by an insect predator in the field.