Parasitic Nematodes And Fusarium Wilt Of Hot Peppers (Capsicum Spp.) And Their Management Using Endospore Forming Rhizo bacteria in Jimma Zone, Southwest Ethiopia

Abstract

Hot pepper is a popular crop in Ethiopia, widely cultivated for consumption and as a significant source of income for smallholder farmers. It also contributes to the overall economy of the country. Among vegetables and spices, pepper ranks first in terms of area under cultivation and production. However, the crop's production and productivity are often constrained by wilt causing pathogens such as Fusarium wilt and root-knot nematode (RKN), particularly during certain seasons. These issues significantly limit Ethiopia's contribution to global markets. Between 2017 and 2022, a comprehensive study was conducted to investigate the occurrence and distribution of plant-parasitic nematodes infecting hot peppers. The study also evaluated the population dynamics and damage threshold of Meloidogyne incognita, assessed the reactions of hot pepper varieties to Fusarium oxysporum and M. incognita, and examined the effectiveness of endospore-forming bacteria from the pepper rhizosphere in controlling these pathogens under both in vitro and in vivo conditions. Soil and plant samples were collected from four major pepper-growing districts in the Jimma Zone. Microbial characterization was performed in the Research and Postgraduate Laboratory of the Department of Biology, while nematode processing, soil physicochemical analyses, and pot experiments were conducted at the College of Agricultural and Veterinary Medicine, Jimma University. Molecular analyses of Meloidogyne isolates were conducted at Ghent University, while those of Fusarium and bacterial isolates were performed at the Swedish University of Agricultural Sciences. Data collected during the study were analyzed using Minitab® version 19, R-4.2.1, SAS version 9.3, and Origin Pro 202. Over 83% of surveyed pepper fields were found to be infested with parasitic nematodes, and 13 nematode types were identified. Among these, Meloidogyne, Scutellonema, Rotylenchulus, Helicotylenchus, Pratylenchus, and Rotylenchus were the most prevalent, with Meloidogyne (identified as M. incognita through Nad5 gene analysis) being the most common. This represents the first report of M. incognita on pepper crops in Ethiopia. The damage potential of M. incognita was investigated through greenhouse experiments, using 13 initial population densities (Pi). The "Dinsire" variety showed a tolerance limit for total fresh weight and fruit number at xvii 0.30 and 0.18 M. incognita (g soil)-1, respectively. Results indicated a negative correlation between Pi and both plant growth and the final nematode population density (Pf). The study estimated the maximum multiplication rate (a) and maximum population density (M) of M. incognita at 8813.2 and 3420.1 M. incognita (g soil)-1, espectively. The "Dinsire" variety performed well at a Pi of fewer than 8 second-stage juveniles (J2) (g soil)−1, exceeding the field average of 6 nematodes (g soil)−1. Fusarium isolates from infected pepper samples were identified as F. oxysporum using ITS region analysis. Among these, the isolate FI1 was highly pathogenic, significantly affecting the "Mareko Fana" variety under greenhouse conditions. Of the 11 varieties tested, "Melka Dera" was resistant to FI1, while "Melka Zala" was highly susceptible, experiencing a 66% reduction in fresh root weight and a 61% reduction in fresh shoot weight. Most pepper varieties currently under production are susceptible to Fusarium wilt, underscoring the need for further screening of genotypes and the development of effective management strategies. From 48 soil samples taken from healthy pepper rhizospheres, 35 endospore-forming bacteria were identified, exhibiting multiple plant growth-promoting traits. These included phosphate solubilization (indices ranging from 2.8 to 10), indole acetic acid production (27.31–59.16 µg/ml), and chitin hydrolysis (chitinolytic indices of 1–7.5 in 20 isolates). Of these, 21 bacterial isolates reduced pathogenic Fusarium growth by 39.4–79.2%, while 12 inhibited M. incognita egg hatching by 51–96.4% and increased nematode J2 mortality by 45–98.7%. Molecular analyses identified these bacteria as Bacillus siamensis, B. velezensis, B. cereus, and Paenibacillus polymyxa. Bacillus cereus (JUBC7) and B. siamensis (JUBC12) demonstrated strong potential for producing plant growth-promoting substances and antagonizing F. oxysporum and M. incognita in vitro. Greenhouse evaluations of 12 pepper genotypes found that the "Dinsire" variety suppressed M. incognita populations, while "Bako Local" and "Dame" were highly susceptible. Co-inoculation of B. cereus and B. siamensis reduced co-infection by M. incognita and F. oxysporum by 92.6%, with B. cereus alone suppressing Fusarium infection by 90.1%. Based on these findings, the "Dinsire" variety, in combination with B. cereus and B. siamensis, is recommended as an integrated pest management strategy for fields infested with Fusarium wilt and M. incognita, pending further validation under field conditions.