1. Fusarium wilt—
Fusarium oxysporum f. sp. lini
Fusarium oxysporum f. sp.
lini is a soil-borne fungus that infects flax plants via the roots. The fungus is host-specific and can impact flax cultivation at early stages, leading to the browning and delayed growth of seedlings or even to the senescence and death of the small flax plants. Infection can also occur in older plants, causing yellow/brown-colored spots on stems, leaves (
Figure 1A) and buds, which subsequently become senescent and die. In an infected field, the impact of the disease is not homogeneous, which is characterized by the presence of brown spots within the field, as shown in
Figure 1B. The apex of infected plants can turn downward, forming a crook.
F. oxysporum colonizes flax xylem vessels, which leads to unilateral water deficiency symptoms that are visible on the side of infected vessels. Disease outbreaks can result in 80 to 100% yield losses, and the pathogen survives for decades in soil as chlamydospores. Isolates of
F. oxysporum f. sp.
lini can differ largely in aggressiveness. The origin of the pathogen is polyphyletic, and isolates from different parts of the world cluster in at least four distinct clonal lineages
[1][2]. The most effective way to manage the disease is the use of resistant cultivars. Most modern flax cultivars show moderate to high resistance to Fusarium wilt. Two QTLs associated with resistance to Fusarium wilt have been identified
[3]. Recently, more insight was obtained into the mechanisms of resistance by a genome-wide association study, and 13 candidate genes involved in Fusarium wilt resistance were identified
[4]. Some soils, such as the Châteaurenard soil in France, are naturally suppressive to Fusarium wilt. This calcic silt-clay soil contains 37.4% of CaCO
3 and has a high pH (7.9). Suppressiveness is caused by the combined action of fluorescent
Pseudomonas bacteria and non-pathogenic
F. oxysporum that compete with pathogenic
F. oxysporum for carbon and iron. Fusarium wilt of flax in a disease-conducive soil could be significantly reduced by the combined application of the non-pathogenic
F. oxysporum strain Fo47 with the phenazine-producing fluorescent
Pseudomonas strain
[5].
Bacillus subtilis subsp.
spizizenii strain ATCC 6633 also has a biocontrol potential against this fungus, and its efficiency to reduce Fusarium wilt of flax has been validated under controlled conditions
[6]. European seed testing protocol
[7] imposes the detection of
Fusarium spp. in less than 5% of tested seeds to guarantee the certification of commercial flax seeds.
2. Scorch—Globisporangium megalacanthum and Berkeleyomyces basicola
Flax scorch is known to occur only in the coastal areas of Northern France, Belgium and the Netherlands
[8]. This disease is characterized by the appearance of glossy lesions on brittle roots, which can lead to tissue necrosis and stunted growth of flax plants (
Figure 1C). Leaves become brown and shriveled with senescence symptoms (
Figure 1D). This disease is mainly due to the combinatorial effect of two pathogens, the oomycete
Globisporangium megalacanthum (previously called
Pythium megalacanthum [8]) and the ascomycete
Berkeleyomyces basicola (previously called
Thielaviopsis basicola or
Chalara elegans [9]).
Globisporangium spp. survive winter as resting structures called oospores.
B. basicola produces chains of dark-colored chlamydospores and is known to cause black root rot on more than 170 agricultural and ornamental plant species
[10]. A cold and wet climate favors the development of these pathogenic complexes, and late sowing is recommended to decrease the risk of flax scorch in fields. A few flax-scorch-resistant cultivars are commercialized since the early 2000s, and their use is highly recommended for cultivation within coastal areas. Biocontrol strategies using seed coating formulations with
Glomus intraradices, an arbuscular mycorrhizal fungi (AMF), and antagonistic fungal strains (
Trichoderma atroviride) could decrease scorch incidence in flax in greenhouse assays
[11].
3. Sclerotinia Stem Rot—Sclerotinia sclerotiorum
This rot-causing pathogen is characterized by the water-soaking lesions (
Figure 1E), bleaching and shredding of flax stems. The fungal mycelium grows on the surface of the infected stem inside which sclerotia (surviving form) are produced.
Sclerotinia sclerotiorum is a problem for linseed in the UK
[12] and for both flax types in Canada
[13]. It has hundreds of host plants, and sclerotia can survive winter and adverse climatic conditions. Lodging increases the risk of infection, the soil-borne inoculum being more likely to infect fallen flax plants by contact. Therefore, for decades, the best way to avoid Sclerotinia stem rot has been to sow lodging-resistant cultivars
[14]. This fungus is also involved in sunflower and rapeseed stem rot, species where genetic resistance has been recently found
[15][16], opening the way for research on genetic resistance in flax. A biocontrol solution is the use of
Coniothyrium minitans, a mycoparasite of
S. sclerotiorum on various hosts, including lettuce, celery, sunflower, bean, oilseed rape and soybean
[17][18]. Applied directly on the soil surface before seed sowing, it parasitizes sclerotia and impairs pathogen development by degrading fungal cells with cell-wall-degrading enzymes
[19]. A biopesticide containing this mycoparasite is commercially available. Other biocontrol strategies to control
S. sclerotiorum such as antibiosis, induced systemic resistance or hypovirulence mediated by mycoviruses have been intensively investigated and were reviewed by Albert
et al. (2022)
[20].
4. Verticillium wilt—Verticillium dahliae
This vascular pathogen enters the plant through the roots and causes yellowing and senescence of leaves and stems, sometimes only on one half of the plant (
Figure 1F)
[14]. Symptoms appear from the bottom of the plant stem since the fungus is soil-borne.
Verticillium dahliae can be present in soil for decades as long-lasting structures (microsclerotia). Infected flax plants can become senescent earlier than non-infected plants, and most of the time, the disease in the field does not produce any symptoms during the vegetative growth stage
[21]. In addition to the strong persistence of the pathogen in soil, the wide range of host plants makes the primary inoculum pressure very difficult to decrease. In the field, the characteristic symptoms appear at the beginning of the retting process, when harvested plant stalks are spread in soil. At this stage, a gray/blue color appears in the stem of infected plants (
Figure 1G). Microscopic black dots can also be visible, which are microsclerotia-producing spots
[21]. The current increase in the frequency of this vascular disease leads to significant economic losses in flax cultivation since it particularly damages the fiber quality. Given the high survival ability of microsclerotia, the efficient removal of old flax residues, especially stalks and stubbles, and the cleaning of tools after each use are crucial points to avoid pathogen propagation within cultivated areas. Even if the genetic mechanisms sustaining the plant responses are studied and known, no genetic resistance against
V. dahliae has been found so far in flax
[22]. Biocontrol options against this fungus are studied with, for example, the biofumigation of soil in potato fields in Canada
[23] or the use of antagonistic
V. isaacii strains, which have been tested in vitro and in fields to control
V. longisporum, the causal agent of Verticillium wilt in cauliflower
[24]. Efforts are also made to map and quantify
V. dahliae populations in field soils, providing important information about the accurate location and the importance of fungal presence in fields
[25].
5. Seedling Blight/Root Rot—Rhizoctonia solani
Seedling blight is caused by a pathogen complex but predominantly by the basidiomycete
Rhizoctonia solani Kühn (teleomorph:
Thanatephorus cucumeris (A.B. Frank) Donk)
[26]. This disease mainly occurs at the early stage of flax development, inducing typical red to brown lesions on the roots and hypocotyl (
Figure 1H) just below the soil surface
[27]. The seedlings attacked by
R. solani often start to yellow, wilt and shrivel, and severe symptoms lead to the death of the flax plantlets
[28]. The fungus might also attack flax plants after the flowering stage and induces root rot symptoms weakening these older plants
[27]. Injuries of the roots make the plant considerably more susceptible to damage by root rot pathogens (such as
Pythium and
Fusarium species) and cold weather
[28].
Current classification systems divide individual, multinucleate
R. solani strains into 13 different anastomosis groups (AGs), based on hyphal fusion, culture morphology, rDNA-internal transcribed spacer sequences and pathogenicity
[29]. Divergent studies revealed that AG 1, AG 2-1, AG 2-2, AG 4, AG 9 and to a lower degree AG 5 are the most aggressive anastomosis groups inducing seedling blight and/or root rot symptoms on flax
[30][31][32][33][34][35][36][37]. Strains belonging to AG 3 only attack older plants, resulting in limited root rot
[36], and AG 6 and AG 7 do not appear to be pathogenic
[30]. The binucleate
Rhizoctonia AG-E has also been isolated from older flax plants, which showed typical symptoms of root rot
[35].
Due to the ability of
R. solani to survive in the soil for a long time and cause disease in a broad range of plant species, sowing flax after alternate hosts is not recommended (e.g., sugar beets, leguminous crops, which are attacked by the same anastomosis groups as flax
[38]). The disease can be controlled by using a combination of practices such as using high-quality seeds, incorporating a grass crop into the rotation with flax and sowing early in a well-prepared firm bed
[28][39]. Brown-seeded linseed cultivars were found to be more tolerant to
R. solani than yellow-seeded cultivars
[40]. The application of fungicides is widely used for controlling
R. solani on many crops, but the use of seed treatments to control flax seedling blight is not a common practice among flax growers
[41], probably due to limited authorized fungicides.
Figure 1. Symptoms of soil-borne diseases in flax. (A) Field flax leaves with yellowing and brown spots resulting from the vascular infection by Fusarium oxysporum f. sp. lini; bar = 0.5 cm. (B) Flax field affected by Fusarium wilt, as shown by brown areas where plants are infected. (C) In vitro roots from 20-day-old flax plants displaying dark brown scorch (Globisporangium megalacanthum and Berkeleyomyces basicola) (1) lesions; bar = 1 cm. (D) Twenty-day-old flax plantlets with stunted growth caused by scorch; bar = 2 cm. (E) Field flax stems and leaves infected by Sclerotinia sclerotiorum and showing water-soaked lesions and bleached color; bar = 0.5 cm. (F) Greenhouse flax plants (30 days old) with half yellowing and senescent leaves, disposed along a gradient from bottom to the top, resulting from Verticillium wilt infection; bar = 1 cm. (G) Retting flax stems showing blue/gray color and Verticillium dahliae microsclerotia (microscopic black spots); bar = 0.5 cm. (H) Rhizoctonia solani causing shriveling and wilting (circle) of 10-day-old flax plantlet; bar = 1 cm. (Credits: A-E, G-H: Arvalis; F: Julie Moyse).