Mildew Of Basil Research Articles

First Report of Downy Mildew Caused by Peronospora belbahrii on Sweet Basil (Ocimum basilicum) in Arkansas

Basil (Ocimum basilicum) is a specialty crop widely grown commercially and in home gardens in Arkansas. In the United States, downy mildew caused by Peronospora belbahrii was first seen and reported in Florida in 2008 (Roberts et al. 2009) and has since been reported in 42 states plus the District of Columbia (Wyenandt et al. 2015). In 2016, a grower in Fayetteville, Arkansas, reported downy mildew in basil to the online monitoring program (McGrath et al. 2010), but neither morphological nor molecular identification was conducted in Arkansas at that time. In 2017, the Arkansas Plant Health Clinic received samples of basil with necrotic leaves and wilting from three Arkansas counties (Pulaski, Sebastian, and Washington). The symptoms began on lower leaves and progressed from yellowing to complete leaf death. A gray mycelial growth was observed on the underside of the leaves. Brown lesions were also observed on the stems. The morphological characteristics of the pathogen were observed using microscopy. Downy mildew caused by P. belbahrii was diagnosed based on the dichotomous branching, hyaline sporangiophores (243 to 620 × 4 to 9 μm), two to five times branched, bearing single sporangia. Sporangia were light brown, ovoid to slightly ellipsoid, and averaged 24 × 20 μm. The morphological identification of P. belbahrii was confirmed with molecular identification. Genomic DNA was extracted from spore suspension from diseased sweet basil tissue with a DNeasy Plant Mini Kit (Qiagen, Germany) according to the manufacturer’s instructions. The rDNA region was amplified with ITS4 and ITS6 primers (Cooke et al. 2000). The consensus sequence was deposited into GenBank (accession no. MG269986). BLAST analysis in the NCBI with products from ITS4 and ITS6 primers revealed 96% similarity with P. belbahrii reported on sweet basil in Florida (GenBank accession no. HQ730979.1), Korea (KX228833.1), China (KP657570.1), Canada (KC756923.1), Czech Republic (KJ960193.1), Hungary (HQ702191.1), and Cyprus (KF419290.1). Leaves of potted basil plants were inoculated by spraying plants until runoff with a suspension containing 1 × 10⁵ sporangia/ml obtained from the sample from Sebastian county. Plants were placed in a dew chamber for 24 h at 22°C and then moved to the greenhouse and maintained at 25°C. Noninoculated plants served as controls and were treated with sterile distilled water. After 12 days, symptoms typical of downy mildew occurred on the inoculated basil plants. Spores and sporangiophores were lifted from the leaf lesions using a tape mount and confirmed as P. belbahrii microscopically. The morphological identification of P. belbahrii was confirmed with molecular identification.

Red Light Increases Suppression of Downy Mildew in Basil by Chemical and Organic Products

AbstractBasil is an economically important herb in the United States and in the world. Recent epidemics of basil downy mildew, caused by Peronospora belbahrii, have significantly affected basil production in the United States. ProPhyt (potassium phosphite), Actigard (acibenzolar‐S‐methyl) and Organocide (sesame oil) were evaluated in the greenhouse in the presence or absence of red light for their effects on the severity of downy mildew and sporangial production by P. belbahrii. Red light at intensity of 12 μmol photons/m2/s significantly (P < 0.05) reduced severity of downy mildew in basil. ProPhyt‐treated basil plants had the lowest disease severity irrespective of red light exposure. Basil plants treated with Actigard and Organocide under red light had significantly lower disease severity compared to plants under dark conditions with the same fungicide treatments 14 and 13 days after inoculation (DAI) in experiments 1 and 2, respectively. Red light significantly reduced AUDPC in the treatments of Actigard and Organocide in both experiments. Basil plants treated with Actigard and Organocide under red light had significantly reduced number of P. belbahrii sporangia than those under dark conditions receiving the same fungicide treatments. This is the first report demonstrating red light in combination with Actigard and Organocide for improved management of downy mildew in greenhouse‐grown basil.

Inheritance of Resistance to Downy Mildew in Sweet Basil

Sweet basil (Ocimum basilicum) is one of the most economically important culinary herbs in the world, yet global production has become increasingly challenging due to the destructive disease downy mildew (Peronospora belbahrii). Although multiple sources of resistance have been identified, there are no resistant sweet basil cultivars with a commercially acceptable chemotype and phenotype available. The commercial basil cultivar Mrihani (MRI) was identified as resistant and crossed with a Rutgers University susceptible sweet basil inbred line (SB22) to generate a full-sibling family. To determine the mode of inheritance for resistance to downy mildew in basil, six related generations of the MRI × SB22 family were evaluated using a disease severity index (DSI) at northern and southern New Jersey locations over 2 years. All siblings in the F1 and BC1P2 generations were resistant (0.33 > DSI) providing strong evidence that inheritance of resistance from MRI was conferred by dominant alleles. Segregation ratios in the F2 and backcross to the susceptible parent (BCP1) generations demonstrated chi-square goodness of fit to the two-gene complementary (F2: P = 0.11, BC1P1: P = 0.04) and recessive epistatic (F2: P = 0.03, BC1P1: P = 0.63) models. Further analyses of gene effects using a weighted six-parameter scaling test provided evidence that nonallelic additive × additive and additive × dominant gene effects were highly significant (P < 0.001) and resistance reducing. This is the first report of heritable genetic resistance that can be introduced to sweet basil without the issue of sterility barriers. Plant breeding strategies using the MRI × SB22 family should exploit dominant gene action and remove recessive, resistance-reducing alleles from the population.

Evaluation of the New Compound Oxathiapiprolin for Control of Downy Mildew in Basil

This study was conducted to evaluate a new chemical, oxathiapiprolin, as a seed treatment and foliar application for control of downy mildew of basil, caused by Peronospora belbahrii, and to assess the in vitro sensitivity of P. belbahrii to oxathiapiprolin. In the greenhouse, both foliar application and seed treatment with oxathiapiprolin significantly (P < 0.05) reduced downy mildew severity on basil and increased width of cotyledons compared to the non-treated control. In field experiments, foliar applications of oxathiapiprolin alone, as a tank mixture, or in alternation with azoxystrobin, potassium phosphite, or tea tree oil significantly reduced downy mildew severity and increased the basil yield compared to the non-treated control. In the laboratory test, EC50 values of sensitive P. belbahrii isolates ranged from 0.00033 μg/ml to 0.00040 μg/ml oxathiapiprolin, providing important information on baseline sensitivity of P. belbahrii to oxathiapiprolin. The results indicate that oxathiapiprolin can be applied as foliar applications in combination or in alternation with azoxystrobin or potassium phosphite to control the disease effectively. Accepted for publication 1 November 2015. Published 13 November 2015.

Rapid staining method to detect and identify downy mildew (Peronospora belbahrii) in basil.

• Premise of the study: Demand for fresh-market sweet basil continues to increase, but in 2009 a new pathogen emerged, threatening commercial field/greenhouse production and leading to high crop losses. This study describes a simple and effective staining method for rapid microscopic detection of basil downy mildew (Peronospora belbahrii) from leaves of basil (Ocimum basilicum).• Methods and Results: Fresh leaf sections infected with P. belbahrii were placed on a microscope slide, cleared with Visikol™, and stained with iodine solution followed by one drop of 70% sulfuric acid. Cell walls of the pathogen were stained with a distinct coloration, providing a high-contrast image between the pathogen and plant.• Conclusions: This new staining method can be used successfully to identify downy mildew in basil, which then can significantly reduce its spread if identified early, coupled with mitigation strategies. This technique can facilitate the control of the disease, without expensive and specialized equipment.

First Report of the Occurrence and Resistance to Mefenoxam of Peronospora belbahrii, Causal Agent of Downy Mildew of Basil (Ocimum basilicum) in Israel.

Downy mildew in basil was first reported from Uganda in 1933 (4). In 2004, it was reported from Italy (3) and, thereafter, from other countries around the world. In Israel, the disease was first observed in November 2011 in two greenhouses located in the northern part of the Jordan Valley. Within a month, second and third outbreaks of the disease occurred simultaneously near the southwest and southeast borders of Israel, 250 km from the initial disease outbreak. By the summer of 2012, the disease had appeared throughout the country, causing major economic damage. The causal agent, identified as Peronospora belbahrii (see below), produced chlorotic lesions on leaf blades with sporangia developing on the lower leaf surfaces. Lesions gradually turn necrotic, and infected leaves abscised. Sporangia were dark purple, oval, 30.4 ± 2.9 μm long × 21.4 ± 1.7 μm wide. Sporangiophores emerged from stomatal openings in a saturated atmosphere, were hyaline, 400 to 600 μm long, dichotomously branched, with three to five branches per sporangiophore, and bore a single sporangium on each branchlet tip. Oospores, seldom seen, were brown, round, and 46.2 ± 2.8 μm in diameter. Sporangia germinated directly, each producing a single germ tube that penetrated the periclinal wall of epidermal cells. PCR assays using sporangia and infected leaves as the template, and specific BAZ primers (1), produced a 134-bp band typical of P. belbahrii (1,2). Twenty isolates, collected from 12 locations in Israel from December 2011 to September 2012, were all sensitive to mefenoxam as the isolates did not cause symptoms on 15-leaf, potted basil plants (cv. Peri, Volcani Center, Israel) that were sprayed with 10 μg mefenoxam/ml (Ridomil Gold 48%, Syngenta, Basel, Switzerland) prior to inoculation. However, one isolate collected in early October 2012 from a severely infected plant in a greenhouse at Rehov in Bet-Shaan Valley, in which the plants had been treated with mefenoxam, was resistant to mefenoxan, showing abundant sporulation on leaves of potted basil plants that had been sprayed with 1,000 μg of mefenoxam/ml prior to inoculation. To our knowledge, this is the first report of the occurrence of downy mildew in basil in Israel. This is also the first global report of resistance to mefenoxam in P. belbahrii.