Phytopythium vexans Associated with Apple and Pear Decline in the Saïss Plain of Morocco

The Rapid Apple Decline Phenomenon: Current Status and Expected Associated Factors in Korea

Rapid apple decline (RAD) is a complex phenomenon affecting cultivated apple trees and particularly dwarf rootstocks on grafted young apple trees. Since its first appearance in the United States, RAD has been reported worldwide, for example in Canada, South America, Africa, and Asia. The phenomenon has also been observed in apple orchards in Korea, and it presented similar symptoms regardless of apple cultivar and cultivation period. Most previous reports have suggested that RAD may be associated with multiple factors, including plant pathogenic infections, abiotic stresses, environmental conditions, and the susceptibility of trees to cold injury during winter. However, RAD was observed to be more severe and affect more frequently apple trees on the Malling series dwarf rootstock. In this study, we reviewed the current status of RAD worldwide and surveyed biotic and abiotic factors that are potentially closely related to it in Korea.

Characterization of Phytopythium Species Involved in the Establishment and Development of Kiwifruit Vine Decline Syndrome

Since 2012, the kiwifruit vine decline syndrome (KVDS) has progressively compromised Italian kiwifruit orchards. Different abiotic and biotic factors have been associated with the establishment and development of KVDS. During monitoring of orchards affected by KVDS in north-western Italy during 2016-2019, 71 Phytopythium spp. were isolated. Based on maximum likelihood concatenated phylogeny on the ITS1-5.8S-ITS2 region of the rDNA, large subunit rDNA, and cytochrome oxidase I, isolates were identified as P. vexans (52), P. litorale (10), P. chamaehyphon (7) and P. helicoides (2). Phytopythium litorale and P. helicoides are reported for the first time as agents of KVDS in Italy. To demonstrate pathogenicity and fulfil Koch's postulates, representative isolates of P. vexans, P. litorale, P. chamaehyphon and P. helicoides were inoculated in potted plants. In these trials, waterlogging was applied to stress plant with a temporary anoxia and to favour the production of infective zoospores by the oomycetes. In experiments in vitro, the four species showed the highest growth at 25-30 °C, depending on the media used. P. helicoides was able to grow also at 40 °C. The four species were able to grow in vitro at a pH ranging from 5.0 to 8.0, showing that pH had less effect on growth than temperature. The present study suggests a strong role of different species of Phytopythium in the establishment and development of KVDS. Phytopythium spp. could be favoured by the average increase in soil temperatures during summer, associated with global warming.

Influence of Culture Media and Environmental Factors (Water Potential and Temperature) on Mycelial Growth of Phytopythium vexans (de Bary), the Causal Agent of Dieback Disease in Apple Trees

This study aimed at evaluating the effects of culture media and environmental factors (temperature and water potential (Ψw)) on the growth of the pathogenic fungus Phytopythium vexans (de Bary) associated with root rot and dieback disease in apple trees. Tomato agar, potato dextrose agar (PDA), and soybean agar were the most favourable for rapid mycelial growth, with optimum growth recorded for PDA medium. To determine the environmental conditions that promoted the development of this phytopathogen, the effects of temperature (5–30 °C), water potential (Ψw) (−15.54; −0.67 MPa) (0.89–0.995 aw), and their interaction were evaluated on the in vitro radial growth rates of the five isolates of P. vexans and on their latency phase (time period prior to growth). The results of this study showed that temperature, water potential, and their interaction had significant effects (p < 0.001) on the radial growth rates and latency phases of all tested P. vexans isolates. All isolates were able to grow throughout the temperature range (5 to 30 °C), with the maximum radial growth rate being observed at the highest temperatures, 25–30 °C. Growth was seen to be faster at −0.67 MPa (0.995 aw) at 25 °C and 30 °C. No growth was observed at Ψw < −5.44 MPa (0.96 aw), regardless of the temperature. It was found that the length of the latency phase depended significantly on both environmental factors. The longest latency phases (5 days on average) were recorded at a temperature of 5 °C and Ψw of −0.67 MPa (0.995 aw) and −2.69 MPa (0.98 aw), while the shortest latency phases were observed at a temperature of 30 °C and a Ψw of −0.67 MPa (0.995 aw), with an average of 0.2 days. The findings from this study could help to understand the impact of these environmental factors on the occurrence of diseases caused by P. vexans and more likely to design a reliable preventive control strategy based on the avoidance of conditions that play in favour of the phytopathogen.

Potential for Biological Control of Pythium schmitthenneri Root Rot Disease of Olive Trees (Olea europaea L.) by Antagonistic Bacteria

Several diseases affect the productivity of olive trees, including root rot disease caused by Pythium genera. Chemical fungicides, which are often used to manage this disease, have harmful side effects on humans as well as environmental components. Biological management is a promising control approach that has shown its great potential as an efficient eco-friendly alternative to treating root rot diseases. In the present study, the antagonistic activity of ten bacterial isolates was tested both in vitro and in planta against Pythium schmitthenneri, the causal agent of olive root rot disease. These bacterial isolates belonging to the genera Alcaligenes, Pantoea, Bacillus, Sphingobacterium, and Stenotrophomonas were chosen for their potential antimicrobial effects against many pathogens. Results of the in vitro confrontation bioassay revealed a high reduction of mycelial growth exceeding 80%. The antifungal effect of the volatile organic compounds (VOCs) was observed for all the isolates, with mycelial inhibition rates ranging from 28.37 to 70.32%. Likewise, the bacterial cell-free filtrates showed important inhibition of the mycelial growth of the pathogen. Overall, their efficacy was substantially affected by the nature of the bacterial strains and their modes of action. A greenhouse test was then carried out to validate the in vitro results. Interestingly, two bacterial isolates, Alcaligenes faecalis ACBC1 and Bacillus amyloliquefaciens SF14, were the most successful in managing the disease. Our findings suggested that these two antagonistic bacterial isolates have promising potential as biocontrol agents of olive root rot disease.

First report of apple root rot caused by Phytopythium vexans in China

Apple production is of great economic importance in the fruit industry of China, where Yunnan Province is considered as a major producing area. A survey was conducted to identify apple trees that were problematic from March to November 2020 in Yunnan Province. Symptoms included smaller yellowing leaves, fewer sprouts per branch, browning and necrosis of the roots and lower parts of the stem bark, and wilting. 20% to 45% of apple trees were found infected and randomly scattered in the surveyed orchards. A total of 110 soil samples were collected from the root area of symptomatic apple trees in Tuanjie Town of Kunming City, Zhaotong City, and Malong District of Qujing City in Yunnan Province. Two grams of each soil sample was suspended in 400 ml of sterile water for three days and each soil extract was baited with two apple leaves (Red Fuji's). Following the baiting, those leaves were cut into 10 pieces (5mm×5mm), surface-sterilized with 70% ethanol for 30 seconds, rinsed three times with sterile water, and then air-dried. Each leaf piece was placed in a Petri dish with the oatmeal agar medium containing PCNB 20 mg/ml, rifampicin 20 mg/ml, and then incubated at 25℃ in the dark for 3 days. A mycelial agar plug was picked from the edge of the colonies and transferred to a fresh Potato Dexrose Agar (PDA) plate. Seventy colonies with similar growing characteristics were isolated from the 110 soil samples. Three isolates were retained for further analysis and named XLD8-1, SD1, and YF2. After being cultivated on PDA plates and incubated at 25℃ in the dark for 4 days, their colonies were rose petal-type and white with dense aerial hyphae (Fig 1, A). In ten days of incubation, oogonium measuring 24.55 ± 1.9μm × 20.27 ± 2.3μm and sporangia measuring 21.65 ± 1.3μm × 19.35 ± 1μm were observed (Fig 1, C, D). The total DNA of the isolates was extracted and amplified using three pairs of primers, ITS1/ITS4 (White et al. 1990), LROR/LR7 (LSU) (Vilgalys R, et al. 1990), and FM58/FM66 (COXⅡ) (Martin F N. 2000). The sequences were uploaded to GenBank (Accession No. OL960234, OK037658, OK052604 for ITS, OL960388, OM838413, OM838314 for LSU, and OM962847, OM962848, OM962849 for COXⅡ). ITS sequences of the three isolates (XLD8-1, SD1, YF2) showed 99.87%,99.87%, 99.87% similar to Pp. vexans (Accession No. AB468784, AB468784, and AM701801). LSU sequences of the three isolates showed 99.92%, 99.72%, 100% similar to Pp. vexans (Accession No. EF426541, MT729990, and EF426541). COXⅡ sequences of the three isolates showed 100%, 99.81%, 99.81% similar to Pp. vexans (Accession No. GU133560). Based on the sequence similarity and morphology, the isolates were identified as Phytopythium vexans. Koch's postulates were conducted by wounding the bases of 3 apple seedlings (1-year-old Red Fuji's) with a cork borer. A plug of mycelium of the isolate XLD8-1 grown on PDA plates was placed on each wound (Fig 1, B). Controls were set up to use sterile agar plugs as an inoculum. Seedlings have incubated an incubator at 23-26°C under the alternating light and dark intervals, 12-hours of each. In 15 days, after were inoculated with XLD8-1 the roots and lower part of the stem bark of those seedlings became brownish and necrotic, and their epidermis was easily sloughed off (Fig 1, E-G). The pathogen isolated from the necrotic root tissues were identical to the isolate XLD8-1. Symptoms of apple growth decline caused by Pp. vexans were reported in Morocco (Jabiri Salma, et al. 2021). This experiment verified that Pp. vexans causes root rot of apple. In China, Fusarium sp. is usually considered the main pathogen causing apple root rot. However, the discovery of large numbers of apple trees that were infected by Pp. vexans in Yunnan Province and the confirmation of pathogenicity of Pp. vexans on apple seedlings have demonstrated for the first time that Pp. vexans could cause apple root rot as Fusarium spp does and become an incoming threat to the apple industry, which lays the foundation for study on the disease epidemiology and integrated management of apple root rot in China. References: Jabiri Salma, et al. 2021. Microorganisms, doi:10.3390/MICROORGANISMS9091916. Martin, F. N. 2000. Mycologia, 92(4), 711-727. Vilgalys R., et al. 1990. Journal of Bacteriology, 172:4238-4246 White, T. J., et al. 1990. PCR Protocols: a guide to methods and applications, 18: 315.