Peronosporaceae species causing downy mildew diseases of Poaceae, including nomenclature revisions and diagnostic resources.

Downy mildew pathogens of graminicolous hosts (Poaceae) are members of eight morphologically and phylogenetically distinct genera in the Peronosporaceae (Oomycota, Peronosporales). Graminicolous downy mildews (GDMs) cause severe losses in crops such as maize, millets, sorghum, and sugarcane in many parts of the world, especially in tropical climates. In countries where the most destructive GDMs are not endemic, these organisms are often designated as high-risk foreign pathogens and subject to oversight and quarantine by regulatory officials. Thus, there is a need to reliably and accurately identify the causal organisms. This paper provides an overview of the Peronosporaceae species causing graminicolous downy mildew diseases, with a description of their impact on agriculture and the environment, along with brief summaries of the nomenclatural and taxonomic issues surrounding these taxa. Key diagnostic characters are summarized, including DNA sequence data for types and/or voucher specimens, morphological features, and new illustrations. New sequence data for cox2 and 28S rDNA markers are provided from the type specimens of three species, Peronosclerospora philippinensis, Sclerospora iseilematis, and Sclerospora northii. Thirty-nine species of graminicolous downy mildews are accepted, and seven previously invalidly published taxa are validated. Fifty-five specimens are formally designated as types, including lectotypification of 10 species, neotypification of three species, and holotype designation for Sclerophthora cryophila. Citation: Crouch JA, Davis WJ, Shishkoff N, Castroagudín VL, Martin F, Michelmore R, Thines M (2022). Peronosporaceae species causing downy mildew diseases of Poaceae, including nomenclature revisions and diagnostic resources. Fungal Systematics and Evolution 9: 43-86. doi: 10.3114/fuse.2022.09.05.

Infectivity and Inoculum Production of Phytophthora ramorum on Roots of Eastern United States Oak Species.

Little is known about colonization of roots of trees by Phytophthora ramorum. We examined zoospore concentration and exposure time needed to infect six Quercus (oak) species and the inoculum produced from their roots. Sprouted acorns, exposed to zoospores (3,000/ml) for different times and transplanted to potting soil, were susceptible to infection within 1 h of exposure but root weights were not impacted after 4 weeks (P = 0.952). Roots of Quercus prinus seedlings, inoculated with sporangia, had 0.6 to 3.2% colonization of the total root mass after 5 months. Neither root lesions nor obvious root sloughing were observed. Inoculum threshold levels were tested by exposing radicles to varying zoospore concentrations for 24 h. Results showed that radicle infection occurred even at 1 zoospore/ml. To test inoculum production, roots were inoculated with sporangia and transplanted into pots. Periodically, samples of runoff were collected and plated on selective medium. Afterward, root segments were plated to calculate percent colonization. After 16 and 35 days, root colonization and inoculum production from oak was lower than that of Viburnum tinus, a positive control. This study shows that P. ramorum is able to infect sprouted oak acorns and produce secondary inoculum, which may be important epidemiologically.

First Report of Powdery Mildew Caused by Erysiphe viburni on Viburnum tinus in California.

In June 2003, landscape and potted nursery plants of laurustinus (Viburnum tinus) in Monterey County, California, were found to be infected with a powdery mildew. White, ectophytic mycelial and conidial growth were present primarily on adaxial leaf sides with only sparse growth on abaxial surfaces. Severely infected leaves were buckled and slightly twisted. Affected leaf tissue exhibited slight purple-to-brown discoloration. Appressoria were opposite and lobed. Conidia were produced singly, cylindrical in shape, and measured 31 to 42 × 14 to 19 µm. No fibrosin bodies were observed in the conidia, and the conidia germinated at the ends. Ascomata were not observed. The fungus was identified as Erysiphe (section Microsphaera) viburni Duby (= Microsphaera sparsa Howe = Microsphaera penicillata [M. sparsa is not completely synonymous with M. penicillata because M. sparsa is defined as only those mildews that attack viburnum, whereas M. penicillata was defined as attacking dogwood, alder, etc.]) (1,2). Pathogenicity was demonstrated by gently pressing infected leaves with abundant sporulation onto recently expanded leaves of four, large, potted laurustinus in standard 5-gallon nursery containers (19 liters). Twelve leaves per plant were inoculated. The plants were incubated in a greenhouse (23 to 25°C). After 9 to 10 days, powdery mildew colonies developed on approximately 50% of the inoculated leaves. Such colonies were morphologically similar to the original fungus. Uninoculated control plants did not develop powdery mildew. Using the same technique, inoculating fully mature leaves of the same plants did not result in disease. To our knowledge, this is the first report of Erysiphe viburni infecting V. tinus in California. At some landscaped areas the powdery mildew was extremely severe, causing plants to take on a whitish appearance and resulting in all new foliage being misshapen. E. viburni has also been reported to infect V. japonicum in California. References: (1) U. Braun. The Powdery Mildews (Erysiphales) of Europe. Gustave Fischer. New York, 1995. (2) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000.

First Report of the Cucurbit Powdery Mildew Fungus (Podosphaera xanthii) Resistant to Strobilurin Fungicides in the United States.

Resistance to strobilurin fungicides was documented in isolates collected from three fungicide efficacy experiments conducted in research fields in Georgia (GA), North Carolina (NC), and New York (NY). In these fields in 2002, strobilurins (fungicide group 11, quinone outside inhibitors [QoI]) when used alone on a 7-day schedule (use pattern not labeled) did not effectively control cucurbit powdery mildew. Strobilurin efficacy declined dramatically after the second application in New York (3). Efficacy also was reduced in commercial fields in Kentucky and research fields in Arizona, California, Kentucky, Illinois, Michigan, and Virginia in 2002 where strobilurins were used predominantly or exclusively. Isolates were collected on 22 July and 8 and 17 October after the last of four, five, and five applications of strobilurin (trifloxystrobin formulated as Flint or azoxystrobin formulated as Quadris) in experiments conducted by J. D. Moore in Chula, GA, M. McGrath in Riverhead, NY, and G. J. Holmes in Clayton, NC, respectively. A leaf-disk bioassay was used to determine fungicide sensitivity (2). Strobilurin sensitivity was determined using trifloxystrobin at 0, 0.5, 5, 50, and 100 µg/ml. Four of nine NY isolates, 19 of 21 GA isolates, and 13 of 15 NC isolates were resistant to strobilurins (grew well on disks treated with trifloxystrobin at 100 µg/ml). The geometric mean of the azoxystrobin baseline was 0.258 µg/ml for Podosphaera xanthii isolates collected in 1998 and 1999 in North America (4). Poor control with strobilurins under field conditions was associated with reduced sensitivity in vitro. Strobilurin sensitivity appeared to be qualitative as reported elsewhere (1). Two sensitive and three resistant isolates responded similarly when tested in another laboratory using kresoxim-methyl and pyraclostrobin (H. Ypema, personal communication). These findings and experiences elsewhere with QoI-resistant P. xanthii indicate that cross-resistance probably extends among multiple QoI's (1). Strobilurins have been available for commercial use in the United States since 1998, when azoxystrobin received Section 18 registration in some states. Federal registration was granted in March 1999. Strobilurin resistance was detected after 2 years of commercial use elsewhere in the world (1). All isolates tested in the current study were from research fields where selection pressure for resistance could have been higher than in commercial fields where strobilurins are used with demethylation inhibitors (DMIs; fungicide group 3) and contact fungicides in alternation or tank mixtures to prevent or delay resistance development. Resistance in commercial fields will reduce the utility of strobilurins, including those not yet registered, and eliminate an important tool for managing DMI resistance. Strobilurins and DMIs are the only systemic fungicides registered for cucurbit powdery mildew in the United States. Managing DMI resistance may be challenged by multiresistant strains. Strobilurin-resistant isolates also exhibited reduced sensitivity to DMIs, tolerating triadimefon at 50 to 100 µg/ml (2). One suggestion to improve resistance management is to apply a contact fungicide with strobilurins as well as DMIs. References: (1) H. Ishii et al. Phytopathology 91:1166, 2001. (2) M. T. McGrath et al. Plant Dis. 80:697, 1996. (3) M. T. McGrath and N. Shishkoff. Fungic. Nematic. Tests. (In press). (4) G. Olaya et al. Phytopathology (Abstr.) 90 (suppl):S57, 2000.

AQ10 Biofungicide Combined with Chemical Fungicides or AddQ Spray Adjuvant for Control of Cucurbit Powdery Mildew in Detached Leaf Culture.

The biofungicide AQ10, a pelleted formulation of conidia of Ampelomyces quisqualis, did not significantly reduce the size of colonies of the cucurbit powdery mildew (Podosphaera xanthii) in detached squash leaf culture but did reduce the amount of inoculum produced by each colony. No significant reduction in colonization of powdery mildew colonies by AQ10 was observed when it was sprayed in conjunction with the fungicides myclobutanil at 10 µg/ml or triadimefon at 100 µg/ml, suggesting that it is not sensitive to the fungicides at these concentrations. The spray adjuvant AddQ did not increase percent colonization by A. quisqualis but reduced the size of mildew colonies when used alone or with AQ10.

First Occurrence of Powdery Mildew Caused by Leveillula taurica on Pepper in New York.

Powdery mildew was observed for the first time on pepper (Capsicum annuum L.) in western New York in August 1999 and on Long Island, NY, in August 2000. Infected plants were found in commercial fields planted with transplants from Georgia and Florida. Powdery mildew was not found in nearby commercial fields in either year, and it was not found in 2000 in western New York. Symptoms included white sporulation on the undersurfaces of leaves, causing yellow lesions on upper surfaces that turned necrotic and led to premature defoliation. The pathogen was confirmed to be Leveillula taurica (Lév.) G. Arnaud, a species complex that infects more than 1,000 plant species in 74 families, including pepper, tomato and eggplant. Only the Oidiopsis stage was found. Conidia were 47.3 to 74.3 µm × 10.5 to 20.3 µm (average 64.0 × 16.8 µm (N = 71). Symptoms were observed on all cultivars of bell and chili pepper in the Long Island field but not on tomato (Lycopersicon esculentum) and eggplant (Solanum melongena var. esculentum) in adjacent rows. Powdery mildew of pepper was first observed in North America in 1971 in southwest Florida (1). Symptoms were found on field-grown peppers in Florida in April 2001 at the time that transplants were being produced for New York. Considering the latent period is 18 to 21 days and symptoms tend to be initially subtle, diseased seedlings could easily go undetected. This disease is a problem on tomatoes and peppers in California (2), Arizona, Utah, and Nevada. Powdery mildew of pepper was reported in Puerto Rico in 1992, in Idaho on greenhouse-grown pepper in 1998, in north-central Mexico in 1998, and in both Canada and Oklahoma on greenhouse-grown pepper in 1999. Powdery mildew of peppers has not been seen in Connecticut, Massachusetts, New Jersey, North Carolina, or Ohio. References: (1) C. H. Blazquez. Phytopathology 66:1155, 1976. (2) R. F. Smith et al. Calif. Agric. 53:40, 1999.

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.

Control of Cucurbit Powdery Mildew with JMS Stylet-Oil.

Laboratory and greenhouse studies were conducted to determine the effect of JMS Stylet-Oil on cucurbit powdery mildew. In laboratory studies, JMS Stylet-Oil (1.5%) applied with a low-pressure sprayer (138 kPa) significantly reduced colony size, but did not eradicate pre-existing mildew colonies. Applying oil every 4 days was more effective than a single application. Oil did not significantly reduce spore viability, since spores taken from sprayed colonies readily formed new colonies. Although oil appeared to cause abnormalities of conidiophores and spores immediately after application, this effect was temporary. Tween 20 had an inhibitory effect on mildew growth and increased the effectiveness of oil when the two were applied together. Efficacy of oil was increased by using a high-pressure sprayer (1380 kPa). In the greenhouse experiment, JMS Stylet-Oil (0.75%) applied once to summer squash either 4 h before transfer of conidia to inoculation sites or 5 days after inoculation suppressed the size of the area infected by 48 to 60%. A single application of oil 5 days before inoculation was not effective. A 4-day spray program beginning 5 days after inoculation was effective. Compared with nontreated plants, oil-treated plants had fewer symptomatic leaves (71 vs. 34%, excluding leaf 1 and inoculated leaves) and fewer colonies (244 vs. 26) 20 days after inoculation.