First Report of Phytophthora nicotianae Causing Dianthus chinensis root rot and foliage blight in China

Dianthus chinensis is a popular ornamental plant that is widely cultivated in China. From May 2020 to 2021, root rot and foliage blight were observed on approximately 50% groundcover plants at several landscape sites of Xuanwuhu Park and Nanjing Railway Station, China. Symptoms of wilting and chlorosis appeared in the initial stage, and severe infection caused the whole plant to die . To recover the causal pathogen, infected root and leaf samples were cut into 5×5 mm2 squares, surface-disinfected in 70% ethanol for 30 sec, placed onto 10% clarified V8 PARP agar at 25°C . After three days, Phytophthora-like hyphae were visibly emerged from both root and leaf tissues and growing into cV8A. Individual hyphal tips were transferred to new cV8A plates to obtain a total of 10 pure isolates. Colony morphology of all isolates on cV8A had slightly radiate to stellate patterns with cottony aerial mycelia. After four or five days all isolates had identical morphological traits including papillate and noncaducous sporangia on cV8, hyphal swellings, and intercalary and terminal chlamydospores. A representative isolate Pni-dc7 was examined for morphological measurements. Sporangia were mostly ovoid and sometimes obpyriform, averaging 28.9±5.6 µm in length and 24.9±5.8 µm in width (n=30). Chlamydospores were abundant and spherical with an average diameter at 29.2 ± 0.3 µm (n=30). Oogonia were not observed. For sequence analysis, the internal transcribed spacer (ITS) regions and large subunit (LSU) of the nuclear ribosomal RNA gene complex were amplified using the primer pairs ITS1/ITS4 and NL1/NL4 , respectively, while the mitochondrial cytochrome c oxidase subunit II (coxII) gene was amplified using FM58/FM66 (Martin et al. 2003). The ITS sequence of isolate Pni-dc7 (GenBank Acc. No. MZ519893) had a 100% identity to those of P. nicotianae (MH219914, KU172524, MT065839). The LSU sequence (MZ573547) had a 100% identity to those of P. nicotianae (KX250514, MZ348950, HQ665198).The cox2 sequence (MZ519893) had a 100% identity to those of P. nicotianae (MH221078, KJ506439, JF707072). Based on morphological and molecular evidence, Pni-dc7 was identified as P. nicotianae. Pathogenicity tests were conducted using both detached leaves and whole plants. Asymptomatic leaves were collected from healthy plants.A 5×5 mm2 Pni-dc7-colonized cV8A plug was placed on each wound of five leaves. Sterile agar plugs were used for a non-inoculated control leaf. All six leaves were placed on a wet filter paper in a closed container at 25°C. All inoculated leaves had necrotic tissues around the wounds, the symptoms progressed from spots to the entire leaves after two days . The control leaves remained asymptomatic. In the whole-plant assay, a D. chinensis plant (approx. 0.3 m in height) was inoculated with 5 mL of zoospore suspension that was mixed into the potting soil(500g). Three plants were inoculated and control plants were treated with sterile distilled water. After two weeks all three inoculated plants in three repeats of the assay had root and crown rot and foliage blight, whereas all control plants remained asymptomatic. P. nicotianae was reisolated from all inoculated plants. This is the first report of P. nicotianae causing root rot and foliage blight on D. chinensis in China. Considering the importance of D. chinensis to both ornamental nursery and landscaping industries in China, diseased plants at the landscape sites were removed to prevent the spread of P. nicotianae to production sites and other landscape locations.

Buckeye rot of tomato in India: Present status, challenges, and future research perspectives

Tomato in India is commonly exposed to various diseases of fungal, bacterial, and viral origin, resulting in substantial yield losses (≥50%). Buckeye rot (caused by Phytophthora nicotianae var. parasitica) is among the major constraints in the successful cultivation of tomato crops in various parts of the world including the Solan district of Himachal Pradesh state, India. The fruit rot becomes more devastating under high humidity and wet soils. Symptoms generally appear on green fruit as alternate dark and light brown concentric rings. The genome size of P. nicotianae var. parasitica is 82 Mb with >23,000 predicted genes. High humidity (> 60%) and optimal temperatures (20-25 ºC), along with rainfall (≥ 10 mm), help to disperse the pathogen as the inoculum reaches the fruit through splashing rains. Sporangia germinate indirectly by producing zoospores at 20-25o C, or directly via germ tubes at >25ºC. In the absence of suitable resistant varieties, no single management practice is sufficient to keep the disease below the economic threshold level; therefore, integration of cultural and chemical methods is preferable. This paper aims to focus on the etiology, and management challenges of buckeye rot. We recommend innovative disease management strategies like identification and deployment of resistant cultivars as well as spraying of synthetic chemical fungicides, biocontrol agents, and use of abiotic chemicals that induce resistance, for developing sustainable crop production practices.

Identification and Management of Phytophthora Aerial Blight Caused by Phytophthora nicotianae on Catharanthus roseus

Phytophthora nicotianae is the most common pathogen in nurseries and gardens, infecting both woody and herbaceous ornamental plants. Phytophthora aerial blight symptoms such dull water-soaked lesions on shoot tips and leaf petioles, girdling on the main stem, necrosis, and wilting of annual vinca were observed in a commercial greenhouse in Warren Co., Tennessee, USA in May 2016. The objective of this study was to identify the causal agent of Phytophthora aerial blight and develop a fungicide management recommendation for ornamental producers. Attempts to isolate the pathogen from symptomatic leaf tissue were conducted and excised leaf pieces were embedded in the V8 agar medium. Morphological characterization, polymerase chain reaction (PCR), sequencing, and pathogenicity test of the isolate FBG2016_444 were conducted to confirm the pathogen identification. The sequence identity was 100% identical to Phytophthora nicotianae, and a combined phylogenetic tree (internal transcribed spacer [ITS]), the large subunit [LSU] of rDNA, and ras-related protein gene [Ypt1]) grouped isolate FBG2016_444 within the clade of P. nicotianae. In the pathogenecity study, all inoculated annual vinca plant showed the Phytophthora aerial blight symptoms and P. nicotianae was re-isolated whereas non-inoculated annual vinca plant remained symptomless. These findings confirmed P. nicotianae as the causal agent of Phytophthora aerial blight of annual vinca. In addition, two rates (0.078 and 0.156 mL·L-1) and three application intervals (7, 14 and 21 days before inoculation [DBI]) of oxathiapiprolin (Segovis®) were evaluated for their ability to reduce the Phytophthora aerial blight severity on annual vinca plants. The control groups were positive (non-treated inoculated) and negative (non-treated non-inoculated) plants. Both rates and application timings of oxathiapiprolin significantly reduced Phytophthora aerial blight severity and disease progress (area under disease progress curve [AUDPC]) on annual vinca plants compared to the positive control. However, 0.078 and 0.156 mL·L-1 of oxathiapiprolin applied at 7 or 14 DBI were the most effective treatments in reducing the disease severity and AUDPC on annual vinca plants. The plant growth parameters such as increase in height and width, total plant weight, and root weight were not influenced by the application of oxathiapiprolin. The finding reported in this study will help ornamental growers with better management of Phytophthora aerial blight of annual vinca.

Gypsum Supplies Calcium to Ultisol Soil and Its Effect on Pineapple Growth, Yield and Fruit Quality in Lower Single Bed under Climate Change Issue

A lower bed single row for pineapple cultivation could protect pineapple from soil erosion in rainy season and during drought, however, disease problem could arise due to water logging. Two experiments using a lower bed single row was done to understand the ability of gypsum providing soil calcium (Ca) available to pineapple plant, resistance to heart rot disease, and give better effect on crop growth and fruit quality of the pineapple in Ultisol soil. In the first trial, four level dosis of gypsum (0, 1.0, 1.5, 2.0 Mg ha-1) and dolomite 2 Mg ha-1 were applied by spreading and incorporated into the soil which have saturated with inoculums of Phytophthora nicotianae. In the second trial, gypsum treatments (0, 1.0, 1.5, 2.0, 2.5 Mg ha-1) were applied in the row between the single row beds as a basic fertilizer. The result showed that P. nicotianae attacked the pineapple plants in all treatments at 6 weeks after planting (WAP), and at 10 WAP, the mortality of dolomite treatment reached 63.8%, significantly different than that for gypsum treatments (3.3-14.3%). In the second experiment, gypsum increased plant weight significantly at 3 until 9 months after planting especially when it was applied 1.5-2.5 Mg ha-1. Fruit texture, total soluble solid (TSS), titratable acidity (TA) were not significant different among the treatment but all meet the standards for grades of canned pineapple. Result showed that soil applied gypsum before planting provides soil calcium and met the plant Ca requirement during a period of early and fast growth step and safe for heart rot disease.

Phytophthora spp.: Economic Plant Threats in Egypt

The potato crop is exposed to infection with many fungal diseases including late blight, caused by Phytophthora infestans. The control of late blight disease requires an integrated management approach represented in cultivation control, plant resistance, and fungicide control. The citrus plants are infected by Phytophthora nicotianae that is causing root rot disease in Egypt. Three species of Phytophthora responsible for infection of citrus plants; P. nicotianae, P. citrophthora, and P. palmivora. Other pathogens associate P. nicotianae and form complexes or coinfection that release different diseases for citrus plants such as gummosis, Phytophthora–Diaprepes complex (PDC), and Huanglongbing syndrome (HLBS).

Evaluation of chemical resistance inducers in maradol papaya against Phytophthora nicotianae var. parasitica

Objective: To test the efficiency of four chemical resistance inducers on Maradol papaya to reduce Phytopthora nicotianae var. parasitica infections in rainfed crops at Chontalpa, Tabasco, Mexico. Design/methodology/approach: Three doses of four resistance inducers were tested on 60-day-old papaya plants in a greenhouse with a randomized design, with four replications and 10 plants as experimental plots. Three days after the inducers' application inoculations with mycelium discs were made, there were negative and positive control treatments to evaluating their efficiency by applying Abbott's formula. Results: The four chemical inducers for resistance (sodium silicate (SS), potassium silicate (PS), potassium phosphite (PF) and acibenzolar-s-methyl (ASM)) were statistically different from the control (P < 0.0001**). The inducers SS 1 %, PS 1 %, FP 0.35 % and ASM 0.1 mM showed higher effectiveness (81.2, 75.9, 74.7 and 74.0 %). Study limitations/implications: The retained effective concentrations were tested in a single application, and their durability is unknown, so this point should be broadened. however, it may be an alternative for repeated use after transplanting. Findings/conclusions: Optimal concentrations of SS, PS, FP, and AMS, that respond against P. nicotianae var. parasitica infections can reduce damages in rainfed crops.

Adaptation of Phytophthora nicotianae to multiple sources of partial-resistance in tobacco.

Host resistance is an important tool in the management of black shank disease of tobacco. While race development leads to rapid loss of single-gene resistance, the adaptation by Phytophthora nicotianae to sources of partial resistance from Beinhart 1000, Florida 301, and the Wz gene region introgressed from Nicotiana rustica is poorly characterized. In greenhouse environments, host genotypes with QTLs conferring resistance from multiple sources were initially inoculated with an aggressive isolate of race 0 or race 1 of P. nicotianae. The most aggressive isolate was selected after each of six host generations to inoculate the next generation of plants. The race 0 isolate demonstrated a continuous gradual increase in disease severity and percent root rot on all sources of resistance except the genotype K 326 Wz/--, where a large increase in both was observed between generations two and three. Adaptation by the race 0 isolate on Beinhart 1000 represents the first report of adaptation to this genotype by P. nicotianae. The race 1 isolate did not exhibit significant increases in aggressiveness over generations, but also exhibited a large increase in aggressiveness on K 326 Wz/-- between generations 3 and 4. Molecular characterization of isolates recovered during selection was completed using ddRADseq, but no polymorphisms were associated with the observed changes in aggressiveness. The rapid adaptation to Wz resistance and the gradual adaptation to other QTLs highlights the need to study the nature of Wz resistance and for conducting field studies on efficacy of resistance-gene rotation for disease management.

First report of Phytophthora nicotianae causing stem canker of Catalpa bungei (Chinese catalpa) in China

Chinese catalpa, Catalpa bungei C.A. Mey is native to China and has been widely cultivated as an important tree species for timber and ornamental purposes (Tao et al. 2019). The properties and high durability of the wood can resist the damage caused by microorganisms and insects (Xiao Y et al. 2019). In September 2020, stem cankers were observed in 5-year-old and 3-year-old C. bungei in a pilot experiment field covering 16-hectare area in Shuyang city (Jiangsu province, China) and in a nursery in Binhai city (Jiangsu Province, China), respectively. The disease incidence in both locations was about 1% to 3%. The typical disease symptoms include small to large, dark-brown and irregular-sunken canker around and along the stem under 2 meters from the stem base. The phloem and xylem of the symptomatic stem were dark brown and the xylem had larger necrosis than the phloem. The cross section of the diseased stem was partially died. The symptomatic stem were collected in both locations for pathogen isolation. In total, seven purified isolates from the diseased samples were obtained using potato dextrose agar (PDA) following standard isolation protocol (Huang et al. 2019). In order to determine the pathogenicity, 3-year-old Chinese catalpa seedlings were artificially inoculated with each of the seven isolates in April 2021. After removing the bark of the stem by a sterilized punch (diameter 6mm), an agar plug (diameter 6mm) pre-colonized by the isolate was inoculated to the stem and the inoculation point was sealed with parafilm. The agar plug without pre-colonization was used as control. Six tree seedlings were inoculated for each isolate. Ten days after inoculation, only the treatment with isolate QS.1 showed obvious discoloration around the inoculation point. One month after inoculation, the canker around the inoculation point was formed (3.4 cm ± 1.0 cm) and spread to the xylem, similar to the symptoms observed in the field. Isolate QS.1 was re-isolated successfully from the inoculated stem based on morphological characters, confirming the Koch's postulates and QS.1 as the causal pathogen. The isolate QS.1 formed white colonies with abundant aerial mycelia on V8 juice agar and produced a large amount of persistent and papillary ovoid sporangia with size of 22 ~ 45μm (average 31μm) × 18 ~ 39μm (average 23μm) in 10% aqueous solution of V8. The spore was spherical with thick-wall and diameter of 24 ± 3.9μm. The morphology of QS.1 is similar to that of Phytophthora nicotianae. The genomic DNA of representative isolate QS.1 was extracted from mycelium by a modified CTAB method (Murray et al. 1980). The rDNA internal transcribed spacer (ITS) region, β-tubulin and EF1-α genes were amplified and sequenced with primers ITS1/ITS4 (White et al. 1990), BTub_F1/TUBUR1 (L. et al. 2004) and EF1A_for/EF1A_rev (Blair et al. 2008), respectively. The BLAST results of these sequences (Accession No. MZ646302, MZ672116, and MZ675589, respectively) showed 99%, 100% and 100% identity with sequences of P. nicotianae (Accession No. KJ494902, KY205750, and MH359041), respectively. Based on the morphological characteristics and DNA analysis, isolate QS.1 was identified as P. nicotianae. To our best knowledge, this is the first report of P. nicotianae causing stem canker on Chinese Catalpa. This disease may pose potential threat on Catalpa due to the increase in Catalpa planting for economic and ecological purposes in China.