Effectiveness of bioactive compounds in amending the soil infested with soil-borne Ralstonia solanacearum of potato

Three natural bioactive compounds, viz. cow dung, bee propolis and turmeric powder were applied to amend the Ralstonia solanacearum infested soil of potato. Sun dried cow dung @ 30 t/ha, aqueous extraction of turmeric powder @ 30 kg/ha and propolis @ 5 lit/ha were applied to the inoculated soil by mixing with field soil in SAU (Dhaka) and BARI (Gazipur). Data on pH, organic matter, total cfu/ml and avirulent cfu/ml of the treatment applied soil and per cent disease index (PDI) of bacterial wilt of potato were recorded. Significant difference in soil pH and per cent organic matter (OM) occurred in all the amended soil as compared to control. A decreased total cfu/ml of R. solanacearum occurred in all the treated soil, but propolis (8.2 X 107) and cow dung (1.1 X 108) showed the lowest count. However, in increasing the avirulent count of R. solanacearum, turmeric powder (1.8 X 109) showed the highest count compared to other amended soil. Significant disease reduction over control was also observed in all the treatments, but propolis (45.65%) and turmeric powder (43.48%) showed the best results in wilt disease reduction. J. Biodivers. Conserv. Bioresour. Manag. 2021, 7(1): 43-50

​Harmful Effect of the Invasive Weed Mikania micrantha with Special Reference to India: A Review

Mikania micrantha is an economically damaging invasive weed, which is widely naturalized in the tropical and subtropical regions of the world. This weed has reported yield losses in various cash crops and plantation crops in South East Asia including India. M. micrantha was found to change the soil microbial communities (bacteria, fungi and actinomycetes) as well as the soil chemical properties. It enhances a significant increase in aerobic bacteria but decreases in anaerobic bacteria due to which infested soil were free from other weeds. Similarly, in soil chemical characteristics significant increases in pH, total N and P were observed and a decrease in soil organic matter where M. micrantha is prevalent. It has allelopathic properties which affect neighbouring crops, weeds, insects and pathogens. This weed produces allelochemicals (phenolics, flavonoids, alkaloids and terpenes) that may be released by volatilization and decomposition of plant debris. It was also found that the aqueous leaf extract of invasive weed M. micrantha inhibited seed germination and seedling growth of various cereals, vegetables, and other plants. A similar result was found in test crops but there was no affected case found in the greenhouse experiment. This review will help the researchers and scientists in understanding the harmful effect of this invasive weed.

Assessing direct and residual effects of cover crops on the soybean cyst nematode, Heterodera glycines

Greenhouse experiments were conducted to determine if cover crops directly decrease population densities of the soybean cyst nematode (SCN), Heterodera glycines, and/or have residual effects on reproduction of the nematode on soybean (Glycine max). Population densities of SCN were not significantly decreased by nine cover crop plants or three cover crop mixes compared to a non-planted soil control in a repeated 60-day-long greenhouse experiment. When susceptible soybeans were grown in the soils after cover crop growth, fewer SCN females formed following three annual ryegrass (Lolium multiflorum) cultivars (Bounty, King, and RootMax), the Daikon radish (Raphanus sativus var. longipinnatus) cultivar CCS779, Kodiak mustard (Brassica juncea), and a mix containing cereal rye, crimson clover (Trifolium incarnatum), plus Daikon radish (cultivars not stated) compared to following the non-planted control. In another repeated experiment, cover crops were grown for 56 days in SCN-infested soil in the greenhouse then exposed to Iowa winter conditions for 28 days to simulate winter termination of the plants. One treatment, a cover crop mix containing Bounty annual ryegrass plus Enricher Daikon radish, had a decrease in SCN population density greater than the non-planted control at the end of the experiment. Significantly fewer SCN females formed on soybeans grown following several cover crops, including the three annual ryegrass cultivars that had the suppressive residual effects in the first experiment. In summary, there were no cover crop treatments that consistently decreased SCN population densities across experiments, and only one cover crop treatment in one experiment significantly reduced SCN population densities more than a non-planted soil control. However, there was a somewhat consistent, adverse, residual effect of cover crops on reproduction of SCN on susceptible soybeans following growth of multiple cover crops.

IMPACT OF PESTICIDE TOLERANT SOIL BACTERIA ON DISEASE CONTROL, PLANT GROWTH PROMOTION AND SYSTEMIC RESISTANCE IN COWPEA

Cowpea, an annual legume, suffers from several disease symptoms caused by Macrophomina phaseolina. Rhizobacteria isolated from pesticide infested soil, identified by blast analysis as Bacillus cereus, Bacillus safensis, Pseudomonas donghuensis and Pseudomonas aeruginosa ascertained tolerant to at least 0.1% pesticides viz. methomyl, imidacloprid and carbendazim. In vitro antagonism against pathogen exhibited maximum by P. aeruginosa 63%. All rhizobacteria were bestowed with attributes responsible for pathogen control and plant growth promotion. Field evaluation resulted highest 75% disease control, enhancement of length, nodule counts, biomass or yield per plant by P. aeruginosa. All rhizobacteria induced systemic resistance in cowpea under challenged inoculation with pathogen by augmenting defensive enzyme production. Highest Phenylalanine Ammonia Lyase activity was expressed in P. aeruginosa treated plants 1.02 μMoles/ml/min, Polyphenol Oxidase by P. donghuensis 1.39 μMoles/ml/min, Chitinase by B. cereus 0.745 μMoles/ml/min and 400 percent relative activity of Peroxidase by P. aeruginosa. The rhizobacteria were prospective for plant disease control, growth promotion and as immunity boosters in pesticide and heavy metal infested toxic environment.

Phytophthora Root and Collar Rot of Paulownia, a New Disease for Europe

Paulownia species are fast growing trees native to China, which are being grown in managed plantings in several European countries for the production of wood and biomasses. In 2018, wilting, stunting, leaf yellowing, and collapse, as a consequence of root and crown rot, were observed in around 40% of trees of a 2-year-old planting of Paulownia elongata × P. fortunei in Calabria (Southern Italy). Two species of Phytophthora were consistently recovered from roots, basal stem bark, and rhizosphere soil of symptomatic trees and were identified as Ph. nicotianae and Ph. palmivora on the basis of both morphological characteristics and phylogenetic analysis of rDNA ITS sequences. Koch’s postulates were fulfilled by reproducing the symptoms on potted paulownia saplings transplanted into infested soil or stem-inoculated by wounding. Both Phytophthora species were pathogenic and caused root rot and stem cankers. Even though P. palmivora was the only species recovered from roots of naturally infected plants, in pathogenicity tests through infested soil P. nicotianae was more virulent. This is the first report of Phytophthora root and crown rot of a Paulownia species in Europe. Strategies to prevent this emerging disease include the use of healthy nursery plants, choice of well-drained soils for new plantations, and proper irrigation management.

Short Communication: Latent Detection of Downy Mildew (Peronospora pisi) in Bioassays against Pisum sativum

Downy mildew of peas is caused by the obligate parasite Peronospora pisi, which occurs sporadically throughout temperate pea-growing regions across the world. To screen pea lines against this biotrophic pathogen, a suitable and reproducible in vitro method using living plant material is required. Field screening can be influenced by environmental factors, thus giving variable results. The aim of this study was to develop a method that could reliably be used to screen pea cultivars against P. pisi in a laboratory setting. A range of bioassays were used to test various methods of inoculation, utilizing sporangia and naturally infested soil. Latent infection was achieved by planting seeds in soil collected from a site with a known history of P. pisi infection and directly inoculating young pea plants with sporangia. Out of the 108 plants which survived the experimental period, only two plants expressed visible signs of disease; however, through a two-step nested PCR process we detected latent infection in 24 plants. This research highlights the importance of considering the presence of latent infection when screening pea lines against downy mildew.

Efficacy of disinfectants against Tropical Race 4 causing Fusarium wilt in Cavendish bananas

Fusarium wilt is one of the most damaging diseases of banana that currently puts the global production and food security at risk. The causal agents belong to the genus Fusarium and the soilborne species that is currently threatening Cavendish varieties and many locally important cultivars is F. odoratissimum, colloquially called Tropical Race 4 (TR4), which spreads easily and rapidly within and between farms. Even though limited information exists on the efficacy of disinfectants, they are promoted as a front-line defense against the spread of TR4 in the banana production areas by equipment and human activity. In this study, 13 disinfectants, the majority marketed in the Philippines, were tested for efficacy on TR4 mycelium, conidia, chlamydospores suspensions and chlamydospore-infested soil. They encompass five chemical groups, quaternary ammonium, halogens, alcohols, diamidines and aldehydes, and were tested at a range of concentrations and exposure times. Conidiospores were sensitive to all tested disinfectants at the manufacturer’s suggested rates. Ten disinfectants controlled mycelial growth with the exception of the quaternary ammonium-based disinfectant GUAA. Eleven disinfectants showed adequate efficacy at all tested concentrations towards chlamydospores in suspension, whereas a calcium hypochlorite-based disinfectant lacked efficacy at all concentrations tested. Despite in vitro efficacy, all disinfectants, except the diamidine-based disinfectant Formo, were largely ineffective in treating chlamydospores present in soil (in situ). However, Formo is corrosive to metals, making it less suitable for various applications. The in vitro results illustrate that overall efficacy of disinfectants depends on the fungal propagule, exposure time, and the environment in which they are exposed. We also determined the exposure times of footwear and vehicles in sanitation baths under field conditions and conclude that those significantly deviate from the minimal required contact time under in vitro conditions. Suboptimal quarantine measures, therefore, are likely to contribute to continued local, regional, and international TR4 dissemination.

Identification of Plectosphaerella melonis from cucumber plants in Ukraine

A fungus was isolated from diseased roots of Cucumis sativus grown in greenhouses. The morphological and cultural characteristics of the isolate allowed it to be classified as Plectosphaerella melonis. BLASTn analysis revealed 99% homology of the ITS sequence from the isolate with 14 Acremonium cucurbitacearum and P. melonis isolates, allowing attribution of the isolate to P. melonis (syn. A. cucurbitacearum). Koch’s hypothesis requirements were fulfilled for the isolate. Symptoms on host roots developed after 14 d of growing cucumber plants on infested soil. Plants of the cucumber variety Nizhynskyi 12 were very susceptible at the two leaf growth stage (2 weeks after sowing). Above-ground disease symptoms were absent after 14 d, even with severely diseased roots. This is the first report of P. melonis on C. sativus in Ukraine.

Ceratocystis ficicola causing a serious disease of Ficus carica in Greece

Ceratocystis ficicola causes vascular wilt of fig trees in Japan, invading root systems and the main stems eventually leading to tree death. In surveys from 2018 to 2020 in fig orchards in Greece, this fungus was detected in two separated regions. The fungus was consistently isolated from infected wood and from rhizosphere soil. The isolates were identified based on multi-locus phylogenetic analyses of rpb2, bt1 and tef1 gene regions and detailed morphological characteristics, including comparisons with an ex-type isolate of C. ficicola from Japan. The pathogenicity of Greek isolates was proven on Ficus carica and F. benjamina plants. Ceratocystis ficicola is a soil-borne pathogen, and the occurrence of vascular wilt outbreaks suggest that the pathogen spreads within and between orchards with infested soil and wood debris during ploughing. The pathogen is also spreading in Greece with infected propagation material. This is the first detailed report of C. ficicola outside Japan, and there is concern over potential spread of the pathogen to other Mediterranean countries, where approx. 70% of the world fig production occurs.