Tar Spot: An Understudied Disease Threatening Corn Production in the Americas

Tar spot of corn has been a major foliar disease in several Latin American countries since 1904. In 2015, tar spot was first documented in the United States and has led to significant yield losses of approximately 4.5 million t. Tar spot is caused by an obligate pathogen, Phyllachora maydis, and thus requires a living host to grow and reproduce. Due to its obligate nature, biological and epidemiological studies are limited and impact of disease in corn production has been understudied. Here we present the current literature and gaps in knowledge of tar spot of corn in the Americas, its etiology, distribution, impact and known management strategies as a resource for understanding the pathosystem. This will in tern guide current and future research and aid in the development of effective management strategies for this disease.

First Report of Downy Mildew Caused by a Peronospora sp. on Basil in Florida and the United States

Basil is grown as a specialty crop in greenhouse and field production in Florida and other regions of the United States. Downy mildew on basil (Ocimum basilicum) was detected from four production sites (Collier, Hendry, Miami-Dade, and Palm Beach counties) in south Florida in the fall of 2007, and within months, it was also found in west-central north Florida (Hillsborough County). Incidence reached nearly 100% on some of the affected crops and caused complete yield losses on basil grown both in the field for fresh market and potted herbs market. Symptoms developed during transit on basil that appeared symptomless at harvest. Symptoms initially appeared as yellowing on the lower leaves that was typically delineated by the veins, although in some cases the entire leaf area of the leaf surface was affected. A gray, fuzzy growth was apparent on the abaxial leaf surface. Microscopic observation detected dichotomous branching, hyaline sporangiophores (220 to 750 × 4 to 9 μm) bearing single sporangia. Sporangia were light brown, ovoid to slightly ellipsoid, and measured 14 to 15 × 15 to 18 μm. Oospores were not observed. Leaves of potted basil plants and coleus (Solenostemon scutellarioides) were inoculated with a suspension containing 1 × 10⁵ sporangia/ml and sprayed till runoff (approximately 15 ml per plant) with a hand-held pressurized aerosol canister. Plants were covered with a plastic bag for 24 h and maintained in the greenhouse under ambient conditions. Noninoculated plants served as controls. After 7 days, symptoms typical of downy mildew occurred only on the inoculated basil plants and sporulation was confirmed microscopically. The internal transcribed spacer regions of an isolate collected in Hendry County were sequenced bidirectionally. The consensus sequence was deposited into GenBank (Accession No. FJ346561). Sequence data matched (100% homology) with a Peronospora sp. reported on sweet basil in Switzerland (GenBank Accession No. AY884605) and was similar (99% homology) to an isolate (GenBank Accession No. DQ523586) reported on coleus, although inoculation to coleus failed to confirm pathogenicity on this host. The sequence data also distinguished the isolate from P. lamii (87% homology) previously reported to occur on basil. The pathogen was identified as a Peronospora sp. based on morphological characteristics and sequencing homology (1-3). References: (1) L. Belbahri et al. Mycol. Res. 109:1276, 2005. (2) S. Francis. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 688. CMI, Kew, England, 1981. (3) A. McLeod et al. Plant Dis. 90:1115, 2006.

First Report of a Leaf Spot Disease of Wild Rocket (Diplotaxis tenuifolia) in Florida Caused by Xanthomonas campestris pv. raphani

Wild rocket, also known as wall rocket or sand rocket (Diplotaxis tenuifolia (L.) DC, family Brassicaceae), is grown in Florida as a salad green and herb, especially for addition to "spring mix" and other bagged salad blends. It is similar in texture and flavor to the more widely known garden arugula (Eruca vesicatoria (L.) Cav. subsp. sativa). During the winter vegetable season of 2006-2007, a leaf spot disease causing severe economic loss was seen in commercial fields of wild rocket near the town of Sebastian in Indian River County, FL. Discrete water-soaked to greasy appearing spots were observed on leaf blades that rarely exceeded 1 mm in diameter with some surrounded by a narrow, yellow halo. There was no evidence of marginal V-shaped lesions suggestive of vascular black rot disease caused by Xanthomonas campestris pv. campestris. A bacterium that formed yellow colonies on nutrient agar was consistently isolated from these lesions. Four strains were isolated, purified, and characterized. All strains were strictly aerobic, gram-negative rods. Strains were positive for esculin hydrolysis and gelatin liquefaction and negative for oxidase, nitrate reduction, urease production, fluorescence on King's B medium, and utilization of asparagine as a sole source of carbon and nitrogen. Proteolysis and an alkaline reaction were observed in inoculated tubes of litmus milk. Colonies were bright yellow and mucoid on plates of yeast extract-glucose-calcium carbonate agar. Carbon source utilization, as revealed by the Biolog system, indicated a match to X. campestris. Fatty acid methyl ester analysis indicated a match with Florida library strains of X. campestris pv. raphani (similarity indices of 0.737 to 0.779). Suspensions (2 × 10⁷ CFU/ml in sterile phosphate-buffered saline) of the four wild rocket strains and a strain isolated in 2003 as a cause of a discrete leaf spot disease of cabbage in southern Florida (1) were sprayed onto plants until runoff with a hand-held plastic mist bottle. Pathogenicity of strains was tested in the greenhouse on seven replicate plants of green cabbage cv. Copenhagen Market, salad arugula cv. Rocket, and wild rocket (an unnamed selection propagated from seed provided by a Florida grower). Symptoms appeared within 6 to 7 days. The wild rocket strains and the cabbage strain were pathogenic on all wild rocket and cabbage test plants, producing small, nonvascular leaf spots. No symptoms were seen on salad arugula or on control plants of wild rocket, cabbage, or arugula sprayed with phosphate-buffered saline. The bacterium was reisolated from infected plants and identified as X. campestris pv. raphani, thus completing Koch's postulates. We have classified the bacterial leaf spot pathogen of wild rocket as X. campestris pv. raphani using the nomenclature of Vincente et al. (2) for X. campestris strains producing nonvascular leaf spots on brassicas. This disease problem seems to be related to widespread use of overhead irrigation in the fields where the disease is prevalent. We have not observed the disease in seepage-irrigated fields of wild rocket. References: (1) K. Pernezny et al. Plant Dis. 87:873, 2003. (2) J. G. Vincente et al. Phytopathology 96:735, 2006.

First Report of Peronospora farinosa f. sp. spinaciae Race 5 Causing Downy Mildew on Spinach in Florida

Downy mildew, caused by Peronospora farinosa f. sp. spinaciae, is an economically important disease in most areas where spinach is grown. This disease has become increasingly important in production fields for prepackaged salad mixes where plant densities typically are very high. In Florida, spinach production for these markets has reached approximately 200 ha. Currently, seven physiological races of the downy mildew pathogen have been described (1). Downy mildew was observed in several commercial spinach fields in the Everglades agricultural area of Palm Beach County, Florida in January 2003 on cvs. Unipak 151 and Merlo Nero. Symptoms appeared as chlorotic and necrotic leaf spots. Disease incidence reached approximately 25% in some field locations. Economic losses were significant, since entire plantings in several fields were not harvested as a result of diminished quality. The race of a field isolate recovered from the cv. Unipak 151 was determined following greenhouse inoculation procedures and using differentials outlined by Irish et al (1). Greenhouse inoculation tests were conducted twice. Disease reactions on a U.S. and international set of differentials indicated that the isolate was race 5. To our knowledge, this is the first report of race 5 occurring outside of the California/Arizona spinach production area in the United States. There are commercial spinach lines with resistance to race 5, as well as the other described races (1). References: (1) B. M. Irish et al. Plant Dis. 87:567, 2003.

Natural Infection of Vicia faba by Bidens mottle virus in Florida

In a study to evaluate the potential of Vicia faba (faba bean) as a cover and forage crop for Florida, 60 accessions of faba bean with diverse genetic backgrounds and geographic origins were acquired from the USDA Germplasm Repository in Pullman, WA. The beans were grown south of Lake Okeechobee in Belle Glade, FL, from December 2000 to April 2001. Reddish-brown elliptical lesions first appeared on the leaflets of two of the faba bean plants 10 to 12 weeks after planting. Within 2 weeks of initial symptoms, a mosaic pattern was expressed on the newly emergent leaves of the same plants. After disease expression, new pods aborted, while developing pods became stunted, distorted, and blistered. Potyvirus cylindrical inclusions (CI) were found in leaf strips (1) of the original plants. Viral symptoms were expressed in manually inoculated plants of Chenopodium amaraticolor, C. quinoa, Lactuca sativa, Nicotiana benthamiana, Petunia × hybrida, Verbena × hybrida, Vicia faba, and Zinnia elegans. Inoculated species of Phaseolus and Pisum were not infected. The virus causing the disease was identified as Bidens mottle virus (2) based on host range, characteristic CI in Z. elegans, and homologous lines of precipitation in SDS-immunodiffusion using antiserum to Bidens mottle and a known antigen. Both the primary host of this virus Bidens mottle virus and its aphid vectors are ubiquitous throughout Florida. To our knowledge, this is the first report of Bidens mottle virus infecting a member of the Leguminosae. References: (1) R. G. Christie and J. R. Edwardson. Light and Electron Microscopy of Plant Virus. Monogr. 9, IFAS, University of Florida, 1994. (2) D. E. Purcifull et al. Bidens mottle virus. Descriptions of Plant Viruses. No. 161. CMI/AAB, Surrey, England, 1976.

Occurrence of Bacterial Leaf Spot of Escarole Caused by Pseudomonas cichorii in the Everglades Agricultural Area of Southern Florida

In the fall of 1997, 1998, and 2000, a leaf spot disease of escarole (Cichorium endivia L.) was widespread among commercial plantings in the Everglades Agricultural Area (EAA), south and east of Lake Okeechobee, FL. Symptoms consisted of dry, dark gray-to-black lesions that expanded to ≈4 cm in diameter. Concentric rings were often observed in mature lesions. Growers and scouts in the area consistently identified this disease as Alternaria leaf spot, because the symptoms closely resembled Alternaria leaf spots seen on a number of other vegetables. Prolific bacterial streaming occurred when cut portions of lesions were observed microscopically. A fluorescent bacterium was consistently isolated when a sterile inoculation needle was pushed through lesions. Eight bacterial strains were isolated, restreaked to obtain pure cultures, and characterized. All strains were aerobic, gram-negative rods that were oxidase positive and arginine dihydrolase negative. Negative reactions were recorded for levan formation and rotting of potato slices. All strains utilized glucose, mannitol, and m-tartrate and were negative for sucrose, sorbitol, benzoate, d-arabinose, l-rhamnose, and cellobiose. Results for utilization of D-aspartate were variable. Based on these results, the causal agent of bacterial leaf spot of escarole was identified as Pseudomonas cichorii. Greenhouse-grown plants of escarole, cv. Full Heart, and Cos lettuce, cv. Tall Guzmaine, were mistinoculated with a suspension (10⁷ CFU/ml) of each test strain from escarole and P. cichorii strain Pc28, originally isolated from celery (1). Plants were bagged for 3 days after inoculation. Symptoms characteristic of this disease were evident on escarole inoculated with all test strains and Pc28 6 days after inoculation. Pure cultures of P. cichorii were recovered from lesions on King's B medium. Three test strains produced mild leaf spot symptoms in Cos lettuce, but the symptoms were distinctly different from those associated with the common bacterial leaf spot of lettuce in Florida caused by Xanthomonas campestris pv. vitians (2). To our knowledge, this is the first report of P. cichorii causing this unusual target spot symptom on escarole in the EAA. References: (1) K. Pernezny et al. Plant Dis. 78:917, 1994. (2) K. Pernezny et al. Plant Dis. 79:359, 1995.

Disease Severity and Yield of Sweet Corn Hybrids with Resistance to Northern Leaf Blight

Reactions of supersweet (sh2) sweet corn to northern leaf blight (NLB) and associated yields were evaluated in Belle Glade, Florida and Urbana, Illinois in yield-loss trials, hybrid evaluations, and evaluations of breeding materials. Hybrids differed significantly for NLB in all trials. Severity of NLB ranged from 0 to 66% on 35 sh2 hybrids in yield-loss trials, and from 0 to 60% on 80 sh2 hybrids in hybrid evaluations. NLB ratings ranged from 1 to 9 (approximately 0 to 80% severity) on 375 hybrids and 186 inbred lines in evaluations of breeding materials. Various methods of rating NLB and ratings from multiple dates were highly correlated, with correlation coefficients ranging from 0.76 to 0.98. Yield, measured as weight of ears and number of marketable ears from inoculated plots as a percentage of that from control plots, decreased as disease severity increased. Linear or quadratic regression models explained 31 to 70% of the variation in percent yield as a function of disease severity at harvest. The effects of NLB on yield were limited by NLB-resistance in several hybrids, including CCO 3268, Chieftain, Crisp N Sweet 710A, Day Star, Envy, Forever, GSS 1526, Jupiter, Midship, Prime Plus, Sch 5005, and SummerSweet 7630. Although high levels of partial resistance to NLB were prevalent among 375 new experimental sh2 hybrids and 186 sh2 inbred lines evaluated in 1995, use of the gene HtN may increase in the near future as breeders are incorporating this resistance into new inbreds and hybrids. Breeders and plant pathologists would be wise to continue improving partial resistance to NLB without using the gene HtN in genotypes with adequate levels of partial resistance, because the widespread use of the gene HtN will select for virulent races of Exserohilum turcicum which occur in Florida, or for races with new combinations of virulence.

Bacterial Leaf Spot of Cilantro in Florida

In the 1995 to 1996 winter vegetable season, a leaf spot disease of cilantro (fresh coriander) (Coriandrum sativum L.) was widespread throughout commercial plantings in the Everglades Agricultural Area south and east of Lake Okeechobee. Symptoms first appeared as water-soaked spots ≤1 mm in diameter. These spots became dark brown to black and enlarged up to 2 mm in diameter. No chlorotic haloes developed around the lesions. The disorder was observed in numerous plantings of the cultivar Longstanding at four separate locations. Symptoms were apparent throughout each planting, with disease severity ranging from a few individual spots to numerous lesions covering nearly entire leaflets. Severe outbreaks were correlated with heavy precipitation events. In several instances, disease levels were great enough to render entire plantings totally unmarketable. A non-fluorescent bacterium was consistently isolated on King's medium B when a cooled inoculation needle was pushed through lesions. Six representative strains were chosen for further characterization. All strains were aerobic, gram-negative rods, and were oxidase and arginine dihydrolase negative. Levan was produced, but potato slices were not rotted. Tests for utilization of l-tartrate, l-lactate, and erythritol were negative. Biolog analysis identified all strains of the bacterium as Pseudomonas syringae. Highest similarity indices (0.52 to 0.81) were with P. syringae pv. pisi for four of the six strains. Suspensions of each strain were swab inoculated onto leaves of 4-week-old Longstanding cilantro seedlings in the greenhouse. Control plants were swabbed with sterile water only. Plants were covered with clear polyethylene bags for 72 h. Watersoaked spots were evident on test plants when bags were removed. Typical brown, greasy-looking leaf spots were seen by 6 days after inoculation. Control plants were symp-tomless. In a host-range study, cilantro and the following plants were mist-inoculated with a 10⁷ CFU/ml suspension of each of the six test strains: carrot (Daucus carota L. 'Fancy Pack'), celery (Apium graveolens L. var. dulce (Mill.) Pers., 'June Belle'), garden pea (Pisum sativum L. 'Melting Sugar'), snap bean (Phaseolus vulgaris L. 'Pod Squad'), and onion (Allium cepa L. 'Evergreen Bunching'). Six days after inoculation, characteristic symptoms were evident on the cilantro. Four of the six strains produced a few (less than 10 per plant) bacterial leaf spot symptoms in carrot from which P. syringae was readily recovered. Some necrosis was observed on young, emerging leaves of snap bean. No symptoms were recorded for the other host species or the controls. This disease is similar to one reported on cilantro in California (1) and Germany (2). References: (1) D. A. Cooksey et al. Plant Dis. 75:101, 1991. (2) H. M. Toben and K. Rudolph. J. Phytopathol. 144:169, 1996.