Disease Risk of Potting Media Infested with Phytophthora ramorum Under Nursery Conditions.

Phytophthora ramorum has been found in potting media of containerized plants; however, the role of infested media on disease development under nursery conditions is unknown. This study assesses pathogen survival, sporulation, and infectivity to rhododendron plants in nursery pots with infected leaf litter that were maintained under greenhouse and field conditions. The influence of environmental conditions and irrigation method on disease incidence was also assessed. Infected leaf disks were buried below the soil surface of potted rhododendrons and retrieved at approximately 10-week intervals for up to 66 weeks. Pathogen survival was assessed by either isolation or induction of sporulation in water over three experimental periods. P. ramorum was recovered from infected leaf disks incubated in planted pots for longer than 1 year. Chlamydospores and sporangia formed on hydrated leaf disks but relative production of each spore type varied with the duration of incubation in soil. Root infections were detected after 40 weeks in infested soil. Foliar infections developed on lower leaves but only after spring rain events. Sprinkler irrigation promoted the development of foliar infections; no disease incidence was observed in drip-irrigated plants unless foliage was in direct contact with infested soil. Management implications are discussed.

Disease on Nursery Stock as Affected by Environmental Factors and Seasonal Inoculum Levels of Phytophthora ramorum in Stream Water Used for Irrigation.

A pear bait monitoring system was used to detect and quantify Phytophthora ramorum propagules in streams that flow through woodland areas with sudden oak death in Santa Cruz County, CA from 2001 to 2007. Stream propagules were detected most frequently or occurred in highest concentrations in winter and spring. The stream propagule concentration was characterized with statistical models using temperature and rainfall variables from 2004 to 2007. The highest concentrations of propagules occurred when stream sampling was preceded by about 2 months with low maximum daily temperatures and by 4 days with high rainfall. The occurrence of propagules in streams in the summer was mostly associated with infected leaves from the native host Umbellaria californica that prematurely abscised and fell into the water. When the stream water was used for irrigating rhododendron nursery stock from 2004 to 2007, disease occurred only three times in the two wettest springs (2005 and 2006) on plants sprinkler irrigated with stream water with relatively high concentrations of propagules. Disease incidence was described with a statistical model using the concentration of infective propagules as measured by pear baiting and consecutive hours of leaf wetness measured by electronic sensors at rhododendron height. The concentration of infective propagules was significantly reduced after water was pumped from the stream and applied through sprinklers.

Phytophthora Leaf Spot and Foliar Blight of Pieris japonica Caused by Phytophthora citricola in California.

During 2004, containerized nursery stock of lily-of-the-valley-bush (Pieris japonica 'Flamingo', family Ericaceae) in Santa Cruz County was affected by a foliar disease. Symptoms consisted of large leaf spots, many developing at the leaf tips that ranged in size from 1 to greater than 4 cm in diameter. Spots were dark brown to almost black, generally oval to round, visible from both sides of the leaf, and did not exhibit signs of any pathogen. Lesions typically expanded and affected the entire leaf, leaf petiole, and stems, resulting in blight-like symptoms. Severely affected leaves abscised from the plant. In advanced stages of the disease, the foliage of the plant was killed. These symptoms resembled those caused by the sudden oak death (SOD) pathogen, Phytophthora ramorum (3). A Phytophthora sp. was isolated consistently from symptomatic leaf tissue. However, the species was identified as P. citricola based on morphological traits that included the following: production of semipapillate, noncaducous sporangia that were irregular in shape and occasionally had more than one apex; presence of oospores with paragynous antheridia in single culture; and radiate to slightly petaloid colony morphology (1). P. ramorum and other fungi were not recovered. Pathogenicity of four representative isolates was confirmed by gently abrading the adaxial surfaces of attached leaves with a sterile wire brush, placing a colonized agar plug (5 mm in diameter) on the surface, misting the leaf with sterile water, and then covering the plug with a plastic cap that was secured with a wire clip. Control leaves were treated in the same manner but received sterile agar plugs. Plants were maintained in a greenhouse at 23 to 25°C. After 2 days, all leaves inoculated with the isolates exhibited dark brown lesions and by day 6, lesions measured 3 cm in diameter. P. citricola was reisolated from symptomatic lesions. Sterile plug control leaves developed no symptoms. The test was repeated and the results were similar. To our knowledge, this is the first report of P. citricola causing a foliar disease of Pieris japonica in California. P. citricola has been reported as a pathogen on Pieris spp. in Ohio (2). Our finding is important because P. ramorum causes very similar symptoms on this same host (3). The occurrence of these two foliar Phytophthora spp. on this ornamental plant may complicate P. ramorum field detection during inspections and laboratory confirmations as established by quarantine regulations. References: (1) D. C. Erwin and O. K. Ribeiro. Morphology and Identification of Phytophthora Species. Pages 96-144 in: Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (2) W. W. P. Gerlach et al. Phytopathology 64:1368, 1974. (3) P. W. Tooley et al. Plant Dis. 88:993, 2004.

Downy Mildew Caused by Peronospora radii on Marguerite Daisy (Argyranthemum frutescens) in California.

In California, marguerite daisy (Argyranthemum frutescens [= Chyrsanthemum frutescens]) is an important, commercially grown, perennial flowering plant that is used as a potted plant, cutflower, and landscape plant. For two seasons (2003 and 2004), a downy mildew disease has been affecting marguerite daisy at wholesale container and field cutflower nurseries and retail nurseries in coastal California (Monterey, Santa Cruz, and San Mateo counties). The disease occurred early in the season (January) and continued to infect new foliage throughout the year whenever cool, foggy weather occurred. The disease primarily affected newly expanded young leaves on shoot tips. Such leaves were chlorotic, twisted and bent, and stunted. In some cases, leaflet tips turned black and necrotic. The abaxial sides of affected leaves were heavily colonized by the extensive purplish brown growth of downy mildew. Older, fully expanded foliage was unaffected. Flowers could be infected with the fungus growing on the undersides of petals and resulting in slightly twisted, bent shapes. Symptomatic plants and cutflower stems were unmarketable. Hyaline conidiophores emerged from stomata, branched dichotomously (rarely trichotomously), and had branches ending in slender, curved branchlets that did not have swollen tips. Conidia were slightly brown, ovoid, mostly nonpapillate, and measured 28.5 to 40.0 × 19.0 to 28.0 µm. Oospores were not observed in plant tissue. On the basis of symptoms and morphology of the organism, the pathogen was identified as Peronospora radii (1,2). To prove pathogenicity, plants were spray inoculated with conidial suspensions, incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C).After 18 to 20 days, symptoms and signs of downy mildew developed only on the newest foliage of inoculated plants, and the pathogen morphology matched that of the originally observed pathogen. Untreated control plants did not develop downy mildew. To our knowledge, this is the first report of downy mildew caused by P. radii on marguerite daisy in California and the United States. The pathogen has not been reported on other hosts in California. P. radii is found on marguerite daisy in England, Germany, Israel, Mexico, and the former Yugoslavia (1,2). References: (1) I. S. Ben-Ze'ev et al. Phytoparasitica 15:51, 1987. (2) O. Constantinescu. Sydowia Ann. Mycol. 41:79, 1989.

First Report of Downy Mildew on Delphinium Caused by Peronospora ficariae in California.

Along California's central coast, delphinium (Delphinium species and cultivars) is grown as a cut flower and potted plant. In the spring of 2003, severe downy mildew was observed on various delphinium crops in Santa Cruz County. The disease was detected on cut flower Delphinium 'Volkerfrieden' in a greenhouse and adjacent field production areas. At a different nursery, outdoor, containerized D. grandiflorum 'Blue Butterfly' and Delphinium elatum (Pacific Hybrids) were also infected. Initial symptoms on leaves consisted of light green patches. These areas quickly turned dark green to blackish green and often were delimited by the central vein of the leaf. Purple-gray sporulation of the pathogen could be seen primarily on the abaxial leaf surfaces. Symptoms were most severe on older foliage. As disease progressed, the lower leaves withered and dried up, giving the plant a blighted appearance. Disease was most severe on D. grandiflorum 'Blue Butterfly', in which the disease affected the entire planting and blighted all foliage with the exception of the youngest leaves on the plant's central leader. For this cultivar, the diseased nursery stock had to be discarded, resulting in significant economic loss. Sporangiophores branched dichotomously, with branches ending in slender, slightly curved tips. Sporangia were slightly purplish, ellipsoid to ovoid, and measured 27 to 36 × 19 to 22 µm. Inside the tissue of older symptomatic leaves, extensive numbers of oogonia and oospores were observed. Oospores were globoid, amber to light brown, verruculose, and measured 25 to 27 µm in diameter. The pathogen was identified as Peronospora ficariae (1). Pathogenicity was demonstrated by spraying sporangial suspensions (1.0 × 104 sporangia per ml) onto the leaves of healthy, potted Delphinium elatum (Pacific Hybrids), incubating the plants in a dew chamber at 18°C for 48 h, and then maintaining them in a greenhouse (22 to 24°C). After 10 to 12 days, inoculated plants developed downy mildew symptoms, and the same pathogen sporulated sparsely on the host. If these plants were again placed in the dew chamber for an additional 24 h, leaf lesions sporulated profusely in the following 48 h. Control plants treated with water did not develop any symptoms or signs of downy mildew. To our knowledge, this is the first report of downy mildew caused by P. ficariae on cultivars of delphinium in California. Reference: (1) G. Hall. Mycopathologia 126:51, 1994.

First Report of a Leaf Spot Disease of Bells-of-Ireland (Moluccella laevis) Caused by Cercospora apii in California.

Bells-of-Ireland (Moluccella laevis) (Lamiaceae) is an annual plant that is field planted in coastal California (Santa Cruz County) for commercial cutflower production. In 2001, a new leaf spot disease was found in these commercially grown cutflowers. The disease was most serious in the winter-grown crops in 2001 and 2002, with a few plantings having as much as 100% disease incidence. All other plantings that were surveyed during this time had at least 50% disease. Initial symptoms consisted of gray-green leaf spots. Spots were generally oval in shape, often delimited by the major leaf veins, and later turned tan. Lesions were apparent on both adaxial and abaxial sides of the leaves. A cercosporoid fungus having fasciculate conidiophores, which formed primarily on the abaxial leaf surface, was consistently associated with the spots. Based on morphology and its host, this fungus was initially considered to be Cercospora molucellae Bremer & Petr., which was previously reported on leaves of M. laevis in Turkey (1). However, sequence data obtained from the internal transcribed spacer region (ITS1, ITS2) and the 5.8S gene (STE-U 5110, 5111; GenBank Accession Nos. AY156918 and AY156919) indicated there were no base pair differences between the bells-of-Ireland isolates from California, our own reference isolates of C. apii, as well as GenBank sequences deposited as C. apii. Based on these data, the fungus was subsequently identified as C. apii sensu lato. Pathogenicity was confirmed by spraying a conidial suspension (1.0 × 105 conidia/ml) on leaves of potted bells-of-Ireland plants, incubating the plants in a dew chamber for 24 h, and maintaining them in a greenhouse (23 to 25°C). After 2 weeks, all inoculated plants developed leaf spots that were identical to those observed in the field. C. apii was again associated with all leaf spots. Control plants, which were treated with water, did not develop any symptoms. The test was repeated and the results were similar. To our knowledge this is the first report of C. apii as a pathogen of bells-of-Ireland in California. Reference: (1) C. Chupp. A Monograph of the Fungus Genus Cercospora. Cornell University Press, Ithaca, New York, 1954.

First Occurrence of Downy Mildew on Digitalis purpurea (Common Foxglove), Caused by Peronospora digitalidis, in California and the United States.

In California, Digitalis purpurea (common foxglove) and D. grandiflora (yellow foxglove) are grown as cutflower, potted, and landscape plant commodities. In the spring of 2002, after seasonably wet and cool weather, severe downy mildew was observed on potted common foxglove plants in commercial nurseries in coastal California (Santa Cruz County). Initial symptoms on leaves consisted of light green, rectangular areas that were vein-delimited and measured 2 to 5 × 8 to 12 mm. Such spots later became chlorotic. As disease progressed, chlorotic spots coalesced and turned necrotic. The purple-gray sporulation of the pathogen could be seen primarily on abaxial leaf surfaces. However, in some cases, extensive fungal growth developed on adaxial surfaces of lower leaves. Conidiophores branched dichotomously and measured 278 to 520 µm long from the lower end to the first branches and 11 to 14 µm across at the widest part of the swollen base. Branch ends were slender with curved tips that measured 11 to 22 µm long. Conidia were hyaline, ellipsoid to ovoid, and measured 22 to 31 µm × 17 to 19 µm. Oospores were not observed. The pathogen was identified as Peronospora digitalidis (1,2). Pathogenicity tests were not conducted. However, the consistent association of sporulation with symptoms, the internal hyphal growth giving rise to conidiophores, and the obligate nature of Peronospora pathogens clearly indicated that P. digitalidis was the causal agent of this disease. Field observations indicated that D. purpurea cvs. Alba and Apricot and Foxy hybrids were very susceptible, D. × mertonensis ( = D. grandiflora × D. purpurea) appeared to be moderately susceptible, and D. grandiflora grown in the same area was symptomless. One planting of Foxy hybrid was 100% infected, and the entire lot of several hundred plants was discarded. The disease also was found on Foxy hybrid seedlings grown in propagation greenhouses. To our knowledge, this is the first report of downy mildew caused by P. digitalidis on cultivars of D. purpurea in California and the United States. This disease has been reported previously in Europe, Asia, and New Zealand (1,2). References: (1) G. Hall. Mycopathologia 126:47, 1994. (2) E. H. C. McKenzie and J. M. Dingley. N. Z. J. Bot. 34:263, 1996.

A Blight Disease of Dill in California Caused by Itersonilia perplexans.

In 2000, a foliar disease was observed in commercial field plantings of dill (Anethum graveolens) in coastal California. Initial symptoms consisted of a gray-green discoloration and wilting of the tips of dill leaves. As disease developed, many of the leaves discolored and collapsed, which gave the foliage a blighted appearance and made the leaves unsuitable for harvest. A fungus was consistently isolated from symptomatic leaves. Slow growing colonies were cream-colored, velutinous, and flat with minimal aerial mycelium. Mycelium was hyaline and had clamp connections. Ovoid to subglobose sporogenous cells were abundant and measured 11 to 14 × 12 to 16 µm. Spores were ballistospores that were bilaterally symmetrical, lunate, 16 to 18 × 12 to 13 µm, and germinated with hyphae or secondary ballistospores. The fungus was identified as Itersonilia perplexans Derx (1,2). Pathogenicity was tested by preparing spore suspensions (1 × 104) and spray-inoculating potted dill plants. Plants were placed in a dew chamber for 24 h and then maintained in a greenhouse (23 to 25°C). After 10 days, leaves showed symptoms similar to those observed in the field and the same fungus was reisolated. Control plants sprayed with water did not develop symptoms. Inoculations were repeated and the results were the same. In another experiment, sets of dill plants and flowers of five cultivars of potted florist's chrysanthemum (Dendranthema × grandiflorum [= Chrysanthemum × morifolium]) and one cultivar of Leucanthemum paludosum (= C. paludosum) were spray-inoculated with several dill isolates and incubated as described previously. Dill plants developed symptoms and the fungus was reisolated. However, flowers of the six chrysanthemum cultivars did not develop any petal blight symptoms. Inoculations were repeated and the results were the same. These findings are consistent with previous studies that indicate I. perplexans consists of pathogenic forms that are host-specific to either chrysanthemum and closely related Compositae or to Apiaceae plants such as dill, carrot (Daucus carota subsp. sativus), parsley (Petroselinum crispum), and parsnip (Pastinaca sativa) (1,2,3). To our knowledge, this is the first report of I. perplexans on dill in California, and also appears to be the first record of this disease in North America. References: (1) T. Boekhout. Mycol. Res. 95:135, 1991. (2) T. Boekhout et al. Can. J. Microbiol. 37:188, 1991. (3) A. G. Channon. Ann. Appl. Biol. 51:1, 1963.

Outbreak of Leaf Spot of Saponaria Caused by Alternaria saponariae in California.

Saponaria (Saponaria vaccaria [= Vaccaria hispanica]) is a Caryophyllaceae plant that is grown commercially in California as a cut flower. In 1998, a leaf spot disease devastated the commercially grown saponaria in coastal California. The entire saponaria crop was completely unmarketable because of extensive leaf spotting. Symptoms consisted of circular, brown, necrotic leaf spots with diameters up to 8 mm and concentric zones of lighter and darker tissue. Chlorotic borders developed around the spots. Conidia from leaves were obclavate, usually had 7 transverse and 1 to 4 longitudinal septa, and narrowed gradually toward the apex into a blunt-tipped, unbranched beak cell. The spore body measured 69 to 90 (to 119) × 17 to 21 (to 25) µm, with the distinctive beak cell 17 to 53 µm long. Conidia formed short chains on host tissue. The fungus was identified as Alternaria saponariae (Peck) Neergaard (2). For pathogenicity tests, six representative isolates were grown on V8 juice agar under fluorescent tube lighting. Potted saponaria were sprayed with either conidial concentrations (1 × 10e5 conidia per ml) or water. Plants were incubated in a chamber with a humidifier for 48 h and then maintained in a greenhouse (23 to 25°C). After 14 days, leaf spots similar to the original symptoms developed on all inoculated plants, and the pathogen was reisolated. Plants sprayed with water were symptomless. The experiment was repeated and the results were similar. Using the same isolates and method, we inoculated carnation (Dianthus caryophyllus), sweet William (Dianthus barbatus), and saponaria. However, disease developed only on saponaria. While A. saponariae on saponaria was reported previously in California (1), this is the first report to characterize the pathogen and document that isolates are pathogenic on saponaria but not on other commercial Caryophyllaceae hosts. References: (1) K. F. Baker and L. H. Davis. Plant Dis. Rep. 34:403, 1950. (2) P. Neergaard. Aarsberet. J. E. Ohlsens Enkes Plantepat. Lab. No. 3, 1938.

A New Root and Crown Rot Disease of Heath in California Caused by Cylindrocladium pauciramosum.

Heath (Erica capensis Salter) is a woody, evergreen plant used in Cali-fornia as a landscape shrub or ground cover. In 1997, a new root and crown disease was found in commercial nursery plantings of potted heath. A similar disease was found in 1998 on heath transplants being grown as liners. In both situations, roots were necrotic and crown tissue turned brown. Affected plants became gray-green in color, withered, and died. A Cylindrocladium species was consistently isolated from roots, crowns, and lower stems of symptomatic plants. Isolates were characterized by having penicillate conidiophores terminating in obpyriform to broadly ellipsoidal vesicles. Conidia were hyaline, 1-septate, straight with rounded ends, (30-) 45 to 55 (-60) × (3.5-) 4 to 5 µm, placing it in the Cylindrocladium candelabrum Viégas species complex. Ten single-conidial isolates produced perithecia with viable progeny of Calonectria pauciramosa C.L. Schoch & Crous when mated on carnation leaf agar with tester strains of Cylindrocladium pauciramosum C.L. Schoch & Crous (1). Matings with tester strains of all other species in this complex proved unsuccessful. Pathogenicity of 8 representative isolates was confirmed by applying 3 ml of a conidial suspension (3.0 × 105 conidia per ml) to the crowns of potted, 6-month-old, rooted heath cuttings that were subsequently maintained in a greenhouse (23 to 25°C). After 2 weeks, plant crowns and roots developed symptoms similar to those observed in the field, and plants later wilted and died. C. pauciramosum was reiso-lated from all plants. Control plants, which were treated with water, did not develop any symptoms. The tests were repeated and the results were similar. This is the first report of C. pauciramosum as a pathogen of heath, and the first record of this pathogen from North America. Reference: (1) C. L. Schoch et al. Mycologia 91:286, 1999.

Rust Disease of Oregano and Sweet Marjoram in California.

In 1996 and 1997, a rust disease was detected on commercial, fieldgrown oregano (Origanum vulgare) and sweet marjoram (Origanum majorana) in coastal California. Symptoms on both plants were similar and mostly consisted of small (2 to 5 mm in diameter), circular, brown, necrotic leaf spots that developed cinnamon brown pustules in the center of the spot or in concentric groups on the spot periphery. Pustules sometimes developed without spots. On sweet marjoram, leaf spots were sometimes surrounded by a chlorotic halo. Teliospores were not detected on either host. Ellipsoidal urediniospores measured 22 to 25 µm by 19 to 22µm and contained 2 to 3 germ pores in an equatorial configuration. The rust was identified as Puccinia menthae. Pathogenicity was tested by depositing urediniospores onto leaves of healthy plants and then incubating plants in a humidity chamber for 48 h. Urediniospores from oregano infected Italian (Origanum× majoricum), Sicilian (Origanum × marjorana), trailing (O. prostrata), and Greek (O. heracleoticum) oregano, and sweet marjoram. Urediniospores from sweet marjoram infected sweet marjoram and the one oregano tested, Italian oregano. With all inoculations, both symptoms and fungal fruiting bodies were similar to those observed in the field. Neither the oregano nor the sweet marjoram isolates infected spearmint (Mentha spicata), which is consistent with previous studies (1,2). This is the first report of a rust disease of oregano and sweet marjoram in California. Rust significantly reduced the quality and yield of both crops. References: (1) J. T. Fletcher. Trans. Br. Mycol. Soc. 46:345, 1963. (2) M. Wilson and D. M. Henderson. 1966. British Rust Fungi, Cambridge University Press, Cambridge.

First Occurrence of Downy Mildew of Statice, Caused by Peronospora statices, in California and the Rest of the United States.

In California, hybrid statice (Misty series; Limonium bellidifolium × Limonium latifolium) is grown as a commercial cutflower crop in fields and greenhouses. In 1997, downy mildew was observed on statice plantings in both southern (San Diego County) and central (Monterey and Santa Cruz counties) parts of coastal California. Initial symptoms consisted of light green, irregularly shaped leaf spots that, after a few days, became chlorotic. As disease progressed, chlorotic spots coalesced and turned necrotic, at times resulting in extensive death of leaf tissues. Under favorable conditions, the purple to gray sporulation of the pathogen could be seen on abaxial surfaces of leaves. Conidiophores had main trunks with dichotomous branches and measured 194 to 335 µm in length (mean = 229 µm) from the base to the first branches and 7 to 8 µm across at the widest part. Branch ends were slender with curved tips that measured 5 to 8 µm long. Conidia were ovoid to globose with very short pedicels, and measured 14 to 19 µm × 14 to 17 µm. Conidial surfaces appeared slightly roughened when viewed with a scanning electron microscope. Clearing leaf sections with 10% NaOH (1) revealed the presence of yellow-brown, globose oospores that measured 31 to 47 µm. The pathogen was identified as Peronospora statices (1). Pathogenicity was demonstrated by pressing leaves with abundant sporulation against healthy leaves of test plants (Misty White) and then placing inoculated plants in a humidity chamber. After 10 to 12 days, symptoms similar to those originally observed developed on inoculated plants; after 14 to 16 days, purple fungal growth morphologically similar to the original isolates grew on leaves. Uninoculated control plants did not develop symptoms or signs of downy mildew. This is the first report of downy mildew caused by P. statices on statice in California and the rest of the United States. The disease has also been confirmed on Blue Fantasia (L. bellidifolium × L. perezii). This disease has been reported previously in Italy, The Netherlands, and the United Kingdom (1). Reference: (1) G. S. Hall et al. Eur. J. Plant Pathol. 103:471, 1997.