Twenty-Six Baits Put to the Test in the Hunt for Hidden Plant-Killing Fungi

Beneath every forest floor lies a community of fungal pathogens waiting for the right cue to reveal themselves. Calonectria, a genus of soil-borne fungi that attacks eucalypt plantations, soybean fields, and ornamental plants across the globe, has long frustrated researchers for exactly this reason: the standard method for pulling these organisms out of the soil relies on baits whose effectiveness has rarely been tested. A new study published in IMA Fungus changes that, presenting one of the most comprehensive comparisons of isolation baits ever attempted for the genus and overturning some long-held assumptions about how best to detect it.

Calonectria species are serious business in plant pathology. With roughly 137 species recognized worldwide based on DNA sequence comparisons, the genus includes pathogens capable of infecting at least 611 plant species across 139 plant families. They cause leaf and shoot blight of Eucalyptus, one of the most economically important forestry crops in China, Southeast Asia, and South America, as well as red crown rot of soybean, black pod rot of peanut, and the devastating blight of boxwood ornamentals. The fungi survive in soil as thick-walled chlamydospores and microsclerotia, persisting for long periods even when no symptoms are visible above ground. At least 84 of the known species have been recovered from soils in Asia, Africa, North America, and South America, making reliable soil detection a cornerstone of both disease surveillance and quarantine efforts.

The traditional approach, in use for decades, involves scattering plant tissue or germinating seeds onto moistened soil and waiting for the fungi to sporulate on the bait surface, after which spores can be transferred to laboratory media. Germinating seeds of alfalfa, Medicago sativa, became the de facto standard after decades of use in studies around the world. Yet, as the authors of the new study point out, this convention rests more on habit than on evidence. Earlier bait evaluations were limited to one or a few baits, and most older studies identified isolates by morphology alone, leaving open the question of whether any given bait actually captures the full diversity of Calonectria lurking in a soil sample.

To close that gap, researchers assembled an ambitious experimental design spanning China’s climatic range. They collected forest soil from eight sites in five provinces and autonomous regions: Fujian and Hainan in the subtropical and tropical south, Heilongjiang in the cold temperate northeast, Henan in the warm temperate center, and Xizang in the subtropical west. From plantations and natural forests, they gathered soil from the surface down to a depth of 50 centimeters, ultimately testing a total of 1,040 isolation cups. Each cup received one of 26 baits: seeds of ten plant species, leaves of fourteen species, segments of soybean stem, and, in a nod to a quirky 1960 report, human hair, which a researcher named Griffin once used successfully to isolate the fungus.

The baits were chosen according to strict criteria. Each had either been used successfully in a previous study, come from a plant from which Calonectria had been isolated in nature, or belonged to a plant family known to host more than ten Calonectria species. Seeds had to be small enough, under five millimeters, to fit in an isolation cup and germinate within two days, and all baits needed to work within 15 days. Alfalfa seed was included as the positive control against which everything else would be judged. Every bait was surface disinfested in 75 percent ethanol, placed on the soil surface, and inspected daily under a dissection microscope for the characteristic conidiophores, the spore-bearing structures that signal a successful capture.

The results were striking. Calonectria appeared in 641 of the 1,040 cups, yielding 1,282 single-conidium isolates. Human hair, despite its historical pedigree, trapped nothing at all. Success rates for the remaining 25 baits ranged from 17.5 to 97.5 percent, with leaves of Rhododendron pulchrum outperforming even the alfalfa control. To identify what they had caught, the team sequenced up to four gene regions for each isolate: tef1, tub2, cmdA, and his3. All 1,282 isolates were first genotyped with tef1 and tub2, producing 41 combined genotypes, and 142 representative isolates were then sequenced across all four loci and subjected to maximum likelihood phylogenetic analysis against reference sequences from 57 known species.

The genetic analysis revealed eight species across three species complexes. Calonectria hongkongensis dominated the collection with 370 isolates, or 28.9 percent, followed by Ca. aconidialis at 21.4 percent, Ca. canadiana at 18.2 percent, Ca. pacifica at 14.1 percent, Ca. kyotensis at 12.2 percent, Ca. lichi at 2.7 percent, Ca. chinensis at 1.8 percent, and Ca. pseudoreteaudii at just 0.7 percent. Geography mattered: Ca. hongkongensis appeared only in Hainan and Fujian, Ca. canadiana in Heilongjiang and Henan, and all isolates of Ca. pacifica and Ca. kyotensis came from Fujian. Notably, statistical comparisons showed that bait performance was not significantly influenced by climatic zone, meaning the conclusions should hold across tropical, subtropical, and temperate forests alike.

The clearest pattern to emerge was that leaves beat seeds. The 14 leaf baits yielded an average of 6.5 species and 61 isolates each, compared with 5.1 species and 40.2 isolates for the 10 seed baits. Leaves of Dianthus caryophyllus and Saccharum officinarum each captured all eight species, and multiple-comparison tests confirmed that several leaf baits, including sugarcane, rose, acacia, carnation, camphor, fan palm, weeping fig, and rhododendron, recovered significantly more species and isolates than the weakest seed baits such as Chinese cabbage, zinnia, ryegrass, chia, garden balsam, and sesame. The best and worst performers differed dramatically: carnation leaves yielded 2.7 times more species and 3.8 times more isolates than zinnia seed. Among seeds, Lythrum salicaria, or purple loosestrife, matched alfalfa in the number of species detected and outperformed it for three species by genotype count, marking it as a viable alternative to the standard.

Perhaps the most consequential finding concerns Calonectria pseudoreteaudii, an important Eucalyptus leaf blight pathogen. This species was trapped by only four of the 26 baits, and leaves of Eucalyptus urophylla, the host itself, recovered the most isolates spanning multiple genotypes. The lesson is straightforward: when hunting a specific pathogen, baiting with tissues of its known host plant can dramatically improve detection. For surveying the genus as a whole, the authors recommend combining germinating seed of alfalfa or purple loosestrife with leaves from a variety of plants, including sugarcane, rose, fan palm, acacia, and rhododendron, and, wherever possible, adding foliage of the target host. Given the wide geographic and climatic spread of the soils tested, and the high species and genetic diversity recovered, the researchers argue that this combined baiting strategy should translate well to Calonectria surveys far beyond China’s borders.

Subject of Research: Comparative efficacy of plant baits for isolating soil-borne Calonectria fungi from Chinese forest soils

Article Title: A comparison of twenty-six baits for isolating Calonectria species illustrates differences in their efficacy and relevance

Article References: Zhou, H., Dong, H., Zhou, W., Wingfield, M. J., & Chen, S. (2026). A comparison of twenty-six baits for isolating Calonectria species illustrates differences in their efficacy and relevance. IMA Fungus, 17, Article e190896. https://doi.org/10.3897/imafungus.17.190896

Image Credits: AI Generated

DOI: 10.3897/imafungus.17.190896

Keywords: Calonectria, plant pathology, soil-borne fungi, baiting, forest soils, Eucalyptus, DNA barcoding, phylogenetics, Medicago sativa, fungus isolation, China, plant quarantine