NAME OF THE ORGANISM: Dematophora necatrix {Rosellinia necatrix} (ROSLNE)

Evaluation continues

Not mentioned in PM4/27(1) Malus (1999). Although the first report of Dematophora necatrix causing problems in apple orchards was in the 1980’s in Italy (Pasini et al., 2016) and France (Guillaumin et al., 1982), it was not considered necessary to add this pathogen to PM4/27 in 1999.

Candidate

• Plant: yes (intermediate important)
• Soil: yes (most important)
• Spores: yes (low importance)
The soilborne fungus Rosellinia necatrix is the causal agent of white root rot disease on numerous plant species, including apple. Although the fungus can travel long distances with plant material, plant material is not expected to be the sole source of inoculum. The heterogeneity within the fungus at one location suggests multiple preexisting sources of infection and not movement of infected soil or plant material from a single source (Pasini et al., 2016; Danfy-Yelin et al. 2018). Due to the ability of the fungus to live as a saprophyte, it can survive in soil for long periods on woody debris (Pasini et al., 2016). To avoid infection, one should avoid planting orchards on the sites of former plantations of the main susceptible hosts such as apples, cherries, poplars and tea bushes; limiting irrigation and organic fertilization; and removing stumps and roots as completely as possible, for instance by subsoiling (Plantwise Knowledge Bank, 2021).
In responses to the questionnaire, BE indicated that 'this pathogen is more common in areas with an oceanic climate or a Mediterranean climate, but in the context of climate change, an expansion of its range towards the north is expected'.
In comparison with Armillaria, the Fruit SEWG considered that this pest was more aggressive, making plants for planting being a significant pathway: it frequently attacks and kills very young orchard plants, often during the first year after planting. The fungus is frequent in nurseries, and contamination of orchards from nurseries is probably a common occurrence (CABI, 2021). The use of healthy planting material from nursery plants is crucial to prevent infection: the means to obtain such material include the use of aerated and dry substrates treated with steam or fungicides (EnviroChange, 2012).
The Fruit SEWG commented that good practices in open-ground nursery would be to establish the nursery on virgin soil and rotate, what should allow to produce healthy material.

Yes

Malus is reported to be susceptible to the fungus (Plantwise Knowledge Bank, 2021).
Agarwala and Sharma (1966) estimated losses caused by R. necatrix in apple (Malus sp.) in the state of Himachal Pradesh, India, to be at least $272,000 in 1963 (Conversion based on Officer, 2002). This conservative estimate converts to about $1.6 million dollars present time (Conversion CPI, Williamson, 2002; cited from Ten Hoopen & Krauss, 2006).
In 1997, an Australian study reported that the white root rot was the most soilborne disease of apple in Queensland (Mouton, 2023). The author found that more than half of the orchards were affected, with estimated annual losses of approximately AUD 1 million for the industry.
It is reported as an increasing problem in apple orchards, in particular in Northern Italy (Trentino, Friuli Venezia Giulia) where soil contamination in orchards is revealed by the presence of large spots of weak or dead plants showing important drought-like symptoms (EnviroChange, 2012). The Fruit SEWG noted that when applying good practices, an orchard should not be replanted in the 4-5 years after another orchard: this would greatly limit the risk. A nursery is generally established on virgin soil. However, because of urbanization, this is not always feasible.

Medium

No

Candidate

Young plant may die from infection (Plantwise Knowledge Bank, 2021), which is considered unacceptable. As good nursery practices should already prevent such infections, the Fruit SEWG questioned whether regulation would provide any real additional benefit.

Candidate

Inclusion in a certification scheme.
Testing the initial stock, visual examination for other categories.
Use virgin soil when establishing a nursery.
Use of substrates as growth media treated with steam or fungicides.
Use of Trichoderma atroviride as a biocontrol agent against R. necatrix reduced white root rot incidence on apple seedlings only if treatment was applied at least 1 week before planting (Pasini et al., 2016).

Candidate