Inhibitory effects of components from root exudates of Welsh onion against root knot nematodes

Exploring the Biocontrol Efficacy of Trichoderma spp. against Rigidoporus microporus, the Causal Agent of White Root Rot Disease in Rubber Trees (Hevea brasiliensis)

Rigidoporus microporus, which causes white root rot disease (WRD) in Hevea brasiliensis, is a looming threat to rubber plantation in Malaysia. The current study was conducted to determine and evaluate the efficiency of fungal antagonists (Ascomycota) against R. microporus in rubber trees under laboratory and nursery conditions. A total of 35 fungal isolates established from the rubber tree rhizosphere soil were assessed for their antagonism against R. microporus by the dual culture technique. Trichoderma isolates can inhibit the radial growth of R. microporus by 75% or more in the dual culture test. Strains of T. asperellum, T. koningiopsis, T. spirale, and T. reesei were selected to assess the metabolites involved in their antifungal activity. Results indicated that T. asperellum exhibited an inhibitory effect against R. microporus in both volatile and non-volatile metabolite tests. All Trichoderma isolates were then tested for their ability in producing hydrolytic enzymes such as chitinase, cellulase and glucanase, indole acetic acid (IAA), siderophores production, and phosphate solubilization. From the positive results of the biochemical assays, T. asperellum and T. spirale were selected as the biocontrol candidates to be further tested in vivo against R. microporus. The nursery assessments revealed that rubber tree clone RRIM600 pretreated with only T. asperellum or with the combination of T. asperellum and T. spirale was able to reduce the disease severity index (DSI) and exert higher suppression of R. microporus compared to other pretreated samples, with the average DSI below 30%. Collectively, the present study demonstrates that T. asperellum represents a potential biocontrol agent that should be further explored to control R. microporus infection on rubber trees.

Effects of Cover Cropping on Microbial Communities Associated with Heterodera schachtii and Nematode Virulence

Nematode-resistant cover crops can suppress populations of plant-parasitic nematodes. Samples of a loamy and a sandy loam soil were collected from two sugar beet fields in Lower Saxonia, northern Germany (“Jeinsen” and “Gross Munzel”) to measure the effects of cover cropping on the population genetic structure and infectivity of Heterodera schachtii as well as the composition of soil microbial communities. These fields allowed for a comparison of cover cropping with Brassica species resistant to Heterodera schachtii to fallow. In a series of radish bioassays with H. schachtii populations from Jeinsen and Gross Munzel, ratios of second-stage juveniles in roots per eggs in soil were higher in soil from under Brassica cropping than from under fallow. In denaturing gradient gelelectrophoresis, profiles of the parasitism gene vap1 differed between Brassica and fallow treatments in both populations. At Gross Munzel, microbes of soils and within nematode cysts differed between Brassica and fallow areas. Specifically, the frequency and occurrence of isolates of Pochonia chlamydosporia and Exophiala salmonis were lower within the cysts from Brassica than from fallow treatments. Overall, cover cropping with resistant Brassica species affected the bacteria and fungi infecting the cysts and subsequently, the infectivity of the H. schachtii population. Cover crop effects on nematode virulence (vap1 gene) and microbial colonization of the cysts could affect long-term nematode population dynamics.

Impacts of Root Metabolites on Soil Nematodes

Plant parasitic nematodes cause significant crop damage globally. Currently, many nematicides have been banned or are being phased out in Europe and other parts of the world because of environmental and human health concerns. Therefore, we need to focus on sustainable and alternative methods of nematode control to protect crops. Plant roots contain and release a wide range of bioactive secondary metabolites, many of which are known defense compounds. Hence, profound understanding of the root mediated interactions between plants and plant parasitic nematodes may contribute to efficient control and management of pest nematodes. In this review, we have compiled literature that documents effects of root metabolites on plant parasitic nematodes. These chemical compounds act as either nematode attractants, repellents, hatching stimulants or inhibitors. We have summarized the few studies that describe how root metabolites regulate the expression of nematode genes. As non-herbivorous nematodes contribute to decomposition, nutrient mineralization, microbial community structuring and control of herbivorous insect larvae, we also review the impact of plant metabolites on these non-target organisms.