First Report of Powdery Mildew Caused by Golovinomyces orontii on Corn-Salad in the United States.

Corn-salad or lamb's lettuce (Valerianella locusta) is a specialty leafy green, annual vegetable that is grown commercially in California for use in salads. During the summer (June through August) of 2004, field plantings in coastal California (Monterey County) showed symptoms and signs of a powdery mildew. White, ectophytic mycelia and conidia were present on leaves and petioles. Extensively colonized leaves were slightly twisted and later developed a tan necrosis. Mycelial growth was spread out, flat, sometimes dense, and colonized both sides of the leaf. Growth was more extensive on the upper leaf surfaces. Hyphae were 5 to 7.5 µm wide with nipple-shaped appressoria. Conidiophores were straight and had foot cells that were simple, mostly curved, measured 50 to 65 × 10 to 12.5 µm, and followed by two to three cells of equal size. Conidia developed in short chains. Conidia were cylindric to doliform, measured 27.5 to 32.5 × 15 to 17.5 µm, and lacked fibrosin bodies. Conidial length-to-width ratios were usually less than or equal to 2.0. Conidia germinated at the ends (cichoracearum-type) and had germ tube lengths that ranged from short to 1.0 to 1.5 times the length of the conidium; germ tubes had swollen tips. Cleistothecia were not observed. On the basis of these characteristics, the fungus is identified as Golovinomyces (Erysiphe) orontii(Cast.) Heluta (1). To demonstrate pathogenicity, heavily colonized corn-salad leaves from commercial plantings were collected and gently pressed onto leaves of potted corn-salad plants. Plants were then maintained in a greenhouse (22 to 24°C). After 8 to 10 days, symptoms and signs of powdery mildew developed on the foliage of inoculated plants, and the pathogen morphology matched that of the originally observed pathogen. Noninoculated control plants did not develop powdery mildew. To our knowledge, this is the first report of powdery mildew of corn-salad caused by G. orontii in the United States. This corn-salad disease has been reported from a number of countries in Europe (2). Because the presence of powdery mildew on the harvested foliage made the leaves unmarketable, part of the crop could not be sold. References: (1) U. Braun. Nova Hedwigia 89:1, 1987. (2) U. Braun. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer Verlag, Jena, Germany, 1995.

First Report of Powdery Mildew Caused by Erysiphe heraclei on Chervil in California.

Chervil (Anthriscus cerefolium) is a culinary herb in the Apiaceae family that is grown commercially in California. In 2003 and 2004, shade house plantings of chervil in central coastal California exhibited symptoms and signs of a powdery mildew disease. White, epiphytic mycelia and conidia were present on petioles and leaves. Severely affected leaflets became slightly twisted and bent. Mycelium was amphigenous and effused or in patches. Hyphae measured 5.0 to 7.5 µm in diameter and had lobed appressoria. Conidiophores were straight and had cylindric foot cells measuring 25.0 to 37.5 × 7.5 to 10.0 µm, followed by a longer cell and one to two shorter cells. Conidia formed singly, were cylindric, and measured (37.5-) 40.0 to 50.0 (-52.5) × 15.0 to 17.5 µm. Conidia lacked fibrosin bodies and germinated at the ends with either a very short or a long germ tube forming a lobed appressorium (Polygoni type). No cleistothecia were observed. On the basis of these characteristics, the fungus was identified as Erysiphe heraclei (1). To confirm pathogenicity, severely colonized chervil leaves from a commercial nursery were gently pressed onto leaves of potted chervil plants. Plants were then maintained in a greenhouse (22 to 24°C). After 10 to 12 days, signs of powdery mildew developed on the foliage of inoculated plants, and the pathogen morphology matched that of the originally observed pathogen. Noninoculated control plants did not develop powdery mildew. To investigate the host range of this chervil isolate, parsley (Petroselinum crispum) and celery (Apium graveolens) plants were inoculated in the same way as described. However, after more than 3 weeks, no powdery mildew developed on these two Apiaceae plants. Control chervil plants again developed the disease. To our knowledge, this is the first report of powdery mildew of chervil caused by E. heraclei in California. Disease impact was severe, and in some cases, chervil crops were not harvested. The inability of the chervil isolate to infect two other Apiaceae plants that are listed as hosts of E. heraclei indicates that this pathogen may consist of strains having different host ranges. Reference: (1) U. Braun. Nova Hedwigia 89:1, 1987.

Heteroallelism at the het-c locus contributes to sexual dysfunction in outcrossed strains of Neurospora tetrasperma.

Neurospora tetrasperma is naturally heterokaryotic, with cells possessing haploid nuclei of both a and A mating types. As a result, isolates are self-fertile (pseudohomothallic). Occasional homokaryotic ascospores and conidia arise, however, and they produce strains that are self-sterile and must outcross to complete sexual reproduction. Invariably, laboratory crosses employing sibling a and A strains from the same parental heterokaryon restore the pseudohomothallic, heterokaryotic state. In contrast, outcrosses employing a and A strains from different wild isolates typically result in sexual dysfunction. Diverse sexual dysfunction types have been observed, ranging from complete sterility to reduced viability. We report that one type of dysfunction, characterized by spontaneous loss of the heterokaryotic state upon ascospore germination, can result from the interaction of incompatible alleles at heterokaryon incompatibility loci. Specifically, we demonstrate that homoallelism at the het-c locus in N. tetrasperma is required for heterokaryon stability. Heterokaryon incompatibility therefore provides an obstacle to outcrossing in this species, an observation with important implications for fungal life-cycle evolution.

Analysis of the pdx-1 (snz-1/sno-1) region of the Neurospora crassa genome: correlation of pyridoxine-requiring phenotypes with mutations in two structural genes.

We report the analysis of a 36-kbp region of the Neurospora crassa genome, which contains homologs of two closely linked stationary phase genes, SNZ1 and SNO1, from Saccharomyces cerevisiae. Homologs of SNZ1 encode extremely highly conserved proteins that have been implicated in pyridoxine (vitamin B6) metabolism in the filamentous fungi Cercospora nicotianae and in Aspergillus nidulans. In N. crassa, SNZ and SNO homologs map to the region occupied by pdx-1 (pyridoxine requiring), a gene that has been known for several decades, but which was not sequenced previously. In this study, pyridoxine-requiring mutants of N. crassa were found to possess mutations that disrupt conserved regions in either the SNZ or SNO homolog. Previously, nearly all of these mutants were classified as pdx-1. However, one mutant with a disrupted SNO homolog was at one time designated pdx-2. It now appears appropriate to reserve the pdx-1 designation for the N. crassa SNZ homolog and pdx-2 for the SNO homolog. We further report annotation of the entire 36,030-bp region, which contains at least 12 protein coding genes, supporting a previous conclusion of high gene densities (12,000-13,000 total genes) for N. crassa. Among genes in this region other than SNZ and SNO homologs, there was no evidence of shared function. Four of the genes in this region appear to have been lost from the S. cerevisiae lineage.

First Report of Powdery Mildew Caused by Podosphaera (Sphaerotheca) fusca on Euryops pectinatus in North America.

Gray-leaved Euryops (Euryops pectinatus Cass., Asteraceae) is an evergreen shrub that is widely planted in landscapes in the United States. In the fall of 1999, powdery mildew was observed on E. pectinatus planted in landscapes in Redlands (San Bernardino County), CA. Symptoms consisted only of slight cupping of leaves. Fungal growth was observed on stems, leaves, petioles, and pedicels and was ectophytic and amphigenous. The white mycelium was patchy to effuse. Hyphal appressoria were indistinct (1). Conidiophore foot cells were cylindric and sometimes were tapered toward or constricted at the base. Foot cells measured 30 to 50 by 10 to 12 µm and were followed by one to two shorter cells. Conidia were cylindric to slightly doliform, borne in chains of two to three, and measured 26 to 38 by 14 to 18 µm. Conidial length to width ratios ranged from 1.7 to 2.4. Catenate conidia had crenate edge lines (3). Conidia possessed conspicuous fibrosin bodies and from their sides produced short germ tubes without appressoria. Cleistothecia were not observed. Based on these characters, the fungus was identified as Podosphaera fusca (Fr.) U. Braun & N. Shishkoff (Podosphaera sect. Sphaerotheca) (1,2). Pathogenicity was confirmed by gently pressing diseased leaves onto leaves of healthy E. pectinatus plants. Plants were incubated in a humidity chamber at 22 to 24°C and after 12 to 14 days powdery mildew colonies developed. E. pectinatus cv. Viridis, a cultivar that lacks the extensive pubescence of E. pectinatus, also developed disease when inoculated. This appears to be the first report of powdery mildew on E. pectinatus in North America. A voucher specimen has been deposited into the University of California Herbarium (accession # UC1738635). P. fusca was also observed on cv. Viridis in a nursery in New York in 1999. It is unclear where this pathogen originated. P. fusca parasitizes a large number of asteraceous species including dandelion (Taraxacum officinalis) and sowthistle (Sonchus spp.) weeds, which occur in the area and sometimes are infected with powdery mildew. The Euryops powdery mildew pathogen may be a race that is different than those found on other composites in the United States. The fungus was observed on plants in shaded areas but not on plants in full sun. References: (1) U. Braun. Nova Hedwigia 89:1, 1987. (2) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (3) H. D. Shin and Y. J. La. Mycotaxon 46:445, 1993.

First Report of Powdery Mildew on Nandina domestica (Berberidaceae) in North America.

Nandina domestica Thunb. (heavenly bamboo) is an ornamental plant that is widely planted in landscapes in California and other states. Since 1996, powdery mildew disease has been seen on outdoor landscape N. domestica in various regions of California (Alameda, Monterey, Riverside, and Santa Barbara counties). Symptoms consist of reddening of leaf and stem tissues colonized by the fungus and curling and twisting of infected leaves. The following observations were the same for all collected isolates. White ectophytic mycelium was observed on leaves and petioles. Mycelium on leaves was amphigenous, mostly epiphyllous, and effused or in patches. Hyphal appressoria were nipple-shaped to lobed and sometimes opposite in orientation. Conidiophores were cylindrical, straight, sometimes slightly flexuous, 22 to 32 × 6 to 8 µm in dimension, and followed by one to two shorter cells. Conidia were cylindrical, produced singly, and 27 to 42 × 11.5 to 14 µm in dimension. Fibrosin bodies were not observed. Conidial germ tubes were approximately twice the length of the spore, originated from the ends of the spore, and terminated in simple appressoria. Cleistothecia were not present. Based on these characteristics, the fungus was identified as Microsphaera berberidis (DC) Lév. (1). Pathogenicity was confirmed by gently pressing diseased leaves on leaves of healthy N. domestica plants. Plants were incubated in a humidity chamber at 22 to 24°C, and after 10 to 14 days, powdery mildew colonies developed. A voucher specimen was deposited in the University of California Herbarium (UC 1738622). Additional inoculation experiments showed that four other N. domestica cultivars were susceptible (Compacta Nana, Gulf Stream, Harbour Dwarf, and Royal Princess). Helfer (2) noted several possible candidates for the Nandina powdery mildew pathogen in the United Kingdom. However, due to the conidial characteristics of that fungus and the paucity of character descriptions for the several species mentioned, no species name was given to the Edinburgh isolate. In contrast, the mitosporic characteristics of our isolates fit the description for M. berberidis. This is the first report of powdery mildew on N. domestica in North America. References: (1) U. Braun. Nova Hedwigia 89:1, 1987. (2) S. Helfer. Plant Dis. 79:424, 1995.

Powdery Mildew of Spearmint Caused by Erysiphe orontii in California.

In 1997 and 1998, the white fungal growth of a powdery mildew was observed on leaves and stems of both nursery and landscape spearmint (Mentha spicata) growing in coastal California (Monterey County). Mycelia were conspicuous, amphigenous, and epiphytic with indistinct to nipple-shaped appressoria. Conidiophore foot cells were cylindrical, straight, nonconstricted at the base, 61 to 92 µm × 11 to 14 µm in size, and were followed by 1 to 3 cells. Doliiform conidia, which were borne in chains of at least 3 to 5 conidia, measured 28 to 33 µm × 17 to 22 µm. Catenate conidia had sinuate edge lines. Conidia lacked fibrosin bodies. Upon germination, conidia produced germ tubes that were mostly apically inserted and ended in club-shaped appressoria, which fit Braun's Cichoracearum-type of conidial germination (1). Cleistothecia were not observed. Based on these features, the pathogen was identified as Erysiphe orontii Cast. (1). Pathogenicity was demonstrated by gently pressing diseased leaves onto leaves of potted spearmint, incubating plants in a humidity chamber for 48 h, then maintaining plants in a greenhouse. The powdery mildew that later developed was morphologically identical to the original isolates. While powdery mildew on spearmint has been observed previously in the state, this is the first report of the disease and first characterization of the pathogen for California. Reference: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987.

First Report of Powdery Mildew, Caused by an Oidium sp., on Poinsettia in California.

In December 1996 and January 1997, powdery mildew was observed on potted poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) plants in Monterey County, CA. Mycelia were observed on stems, petioles, mature and immature leaves, and bracts. Severely diseased leaves became twisted and bent and senesced prematurely. The white mycelia were conspicuous, epiphytic, and amphigenous; hyphae measured 4.6 to 6.9 µm in diameter. Growth initially was in patches but eventually became effused. Appressoria were slightly lobed to lobed and sometimes opposite. Conidiophore foot cells were cylindrical, sometimes bent at the base, and slightly flexuous to flexuous. Foot cells measured 30.0 to 46.2 µm × 5.8 to 6.9 µm and were followed by one to two shorter cells. Conidia were cylindrical to slightly doliform and measured 25.4 to 32.3 µm × 11.6 to 18.5 µm. The length-to-width ratios of conidia generally were greater than 2.0. Conidia were produced singly, placing the fungus in the Pseudoidium-type powdery mildew group. Conidia germinated at the ends, and no fibrosin bodies were observed. Cleistothecia were not found. The fungus was identified as an Oidium species. Pathogenicity was demonstrated by gently pressing infected leaves having abundant sporulation onto leaves of potted poinsettia plants (cvs. Freedom Red, Peter Star Marble, and Nutcracker White), incubating the plants in a moist chamber for 48 h, and then maintaining plants in a greenhouse. After 12 to 14 days, powdery mildew colonies developed on the inoculated plants, and the pathogen was morphologically identical to the original isolates. Uninoculated control plants did not develop powdery mildew. This is the first report of powdery mildew on poinsettia in California. This fungus appears similar to Microsphaera euphorbiae but has longer, slightly flexuous foot cells that do not match the description for M. euphorbiae (1,2). An alternative identification would be Erysiphe euphorbiae; however, there are no available mitosporic descriptions for morphological comparisons (1,2). In the United States, powdery mildew of poinsettia previously has been reported in various states in the Pacific Northwest, Midwest, and Northeast. References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN.

First Report of Powdery Mildew Caused by Oidium araliacearum on Ivy in California.

Powdery mildew was observed on English ivy (Hedera helix L.) in Berkeley, CA, in the spring of 1997. Fungal growth was ectophytic and amphigenous on leaves and petioles and caused slight reddening and buckling of leaves. The mycelium was white, effuse to dense, and had hyphal diameters of 3.5 to 6.9 µm. Appressoria were nipple-shaped to lobed, and sometimes were opposite. The foot cells of the conidiophores were cylindric, sometimes slightly flexuous, and were 18.5 to 30.0 × 6.9 to 9.2 µm in diameter. The foot cells were followed by 1 or 2 cells. When only one cell followed the foot cell, this cell was longer and slightly wider than the foot cell and measured 20.8 to 46.2 × 6.9 to 9.2 µm. When two cells followed the foot cell, these cells were of similar length to the foot cell, with the second cell usually shorter. Conidia were formed singly, were cylindric to slightly doliiform, and measured 32.0 to 44.0 × 11.5 to 19.0 µm. Fibrosin bodies were not observed. Conidia germinated at the ends and germ tubes terminated in lobed appressoria, placing this fungus in the Polygoni-type germination category (2). Cleistothecia were not observed. Based on these characters, the fungus was identified as Oidium araliacearum U. Braun & E. Oehrens B. A specimen was deposited with the University of California at Berkeley herbarium (accession number UC1713247). Pathogenicity was confirmed by gently pressing infected leaves onto leaves of various stages of maturity on ivy vines (3 to 4 vines per plant; 4 plants). Inoculated plants were kept in a moist chamber for 48 h, and then maintained in a greenhouse. Powdery mildew developed on inoculated plants, primarily on leaves of intermediate maturity, after 14 days, while uninoculated plants did not develop disease. Phyllactinia guttata is the only other powdery mildew recorded on English ivy (2). O. araliacearum differs from P. guttata in conidial shape (cylindric vs clavate), germination habit (end vs side of conidia), and asexual genus (Oidium vs Ovulariopsis) (2). Previous to this report, O. araliacearum was described only on Pseudopanax valdiviensis, another Araliaceae plant, in Chile (1). This is the first report of a powdery mildew disease of ivy in California, and the first report of Hedera helix as a host of O. araliacearum. References: (1) U. Braun. Mycotaxon 25:259, 1986. (2) U. Braun. Nova Hedwigia 89:1, 1987.

First Report of Powdery Mildew, Caused by Erysiphe cruciferarum, on Broccoli Raab in California.

Broccoli raab (Brassica rapa subsp. rapa), also known as rappini, is a leafy vegetable that is widely grown in Europe and has in recent years become an important crop in coastal California. During summer months in the Salinas Valley (Monterey County) in California, powdery mildew was observed on commercial field and greenhouse-grown plants. White ectophytic mycelial and conidial growth was present on leaves, causing chlorosis, and also developed on stems. Mycelia were amphigenous, in patches, often spreading to become effused. Appressoria were lobed and conidiophores were straight. Foot cells were cylindrical, measured 18 to 26 × 7 to 10 µm, and were followed by one to two cells of variable length. Conidia were produced singly, were cylindrical, and measured 35 to 50 × 12 to 21 µm with a length-to-width ratio greater than 2. No fibrosin bodies were observed in the conidia, and conidia germinated at the ends. Cleistothecia were not observed. The fungus was identified as Erysiphe cruciferarum Opiz ex Junell (1). Pathogenicity was demonstrated by gently pressing infected leaves having abundant sporulation onto leaves of potted broccoli raab (cv. Spring Raab), incubating the plants in a moist chamber for 48 h, and then maintaining plants in a greenhouse. After 10 to 13 days, powdery mildew colonies developed on the test plants. Uninoculated control plants did not develop powdery mildew. With the same inoculation procedure, potted broccoli (Brassica oleracea subsp. botrytis cv. Greenbelt) also became infected by the broccoli raab isolates. This is the first report characterizing this pathogen on broccoli raab in the state. Disease incidence and severity in field-planted crops were low, but greenhouse-grown plants became severely infected. Reference: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987.

First Report of Powdery Mildew Caused by Erysiphe heraclei on Celery in North America.

Since 1994, a fungal disease has been affecting celery (Apium graveolens) grown for seed in the central coast region of California. White, ectophytic mycelia and conidia, characteristic of a powdery mildew, were present on compound umbels, including the involucre, rays, raylets, flowers, and fruits. Celery with powdery mildew had twisted flower petals, and the disease possibly contributed to premature senescence and drying of the fruits. Powdery mildew was not observed on leaves. Mycelial growth was effused and amphigenous and had lobed appressoria. Conidiophores were straight, and cylindrical foot cells were followed by a longer cell and one or two shorter cells. Conidia were produced singly and were 36 to 44 × 11 to 16 µm. Fibrosin bodies were not observed. Germ tubes were located at the ends of conidia and formed lobed appressoria. Cleistothecia were not present. Based on these characteristics, the fungus was identified as Erysiphe heraclei (1). In California, parsley (Petroselinum crispum) is a host of E. heraclei (4), and parsley is grown in the same region as celery seed crops. To determine if parsley was an alternative host of the celery powdery mildew, infected celery umbels were gently pressed onto adaxial surfaces of leaves on 2-month-old greenhouse grown parsley cvs. Italian Plain and Triple Curled. Inoculated plants were incubated 48 h in a moist chamber at 22°C, then transferrred to a greenhouse at 22 ± 2°C / 16 ± 2°C day/night temperatures, 75% relative humidity, and natural light. After 12 to 14 days, powdery mildew was observed on both parsley cultivars. Uninoculated control plants did not develop the disease. This is the first report of powdery mildew on celery in North America. Previous reports list E. heraclei on celery in Chile, the former Czechoslovakia, France, Iraq, Italy, and the former USSR (Georgia region). In the U.S., E. heraclei is reported on carrot (Daucus carota) (2), parsley (4), meadow parsnip (Zizia aptera and Z. aurea) (3), and now celery. References: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN. (3) K. A. Hirata. Host Range and Geographic Distribution of the Powdery Mildew Fungi. Jpn. Scientif. Soc. Press, Tokyo. (4) S. T. Koike and G. S. Saenz. Plant Dis. 78:1219, 1994.