NAME OF THE ORGANISM: Peronospora sparsa {Peronospora rubi} (PERORU)

Evaluation continues

EPPO Standard PM 4-10 Certification scheme for Rubus recommends inspection for Peronospora rubi as appropriate to the Rubus sp. or hybrid concerned. In the responses to the questionnaire, DE and NL supported deregulation in the EU, considering that plants for planting was not the main pathway. Evaluation continues on this criteria. DE considered that 'a generally known pest that can be easily managed by the operator should not be regulated as an RNQP. Horizontal regulation in the marketing regulations should ensure that the consignments to be marketed/moved are “practically free from pests”'.

Candidate by default

Peronospora sparsa has a relatively wide host range. It infects roses and Rubus spp. e.g. Rubus ideaus, Rubus fructicosus, Rubus arcticus, boysenberry (Rubus ×loganobaccus LH Bailey), tummelberry (a cross between tayberry (blackberry cv. Aurora x tetraploid red raspberry) and another tayberry seedling from the same cross), R. cissoides, R. cissburiensis, R. chamaemorus (CABI, 2021, Lindqvist et al., 1998; Gubler & Rebollar-Alviter, 2017; Hall, 1987; Hall & Shaw, 1989; Kaponen et al., 2000; Rebollar-Alviter et al., 2009). Other hosts within the family of Rosaceae have also been mentioned for P. sparsa: Prunus lauracerasus, Actea and Fragaria (Hall et al., 1992; Constantinescu, 1996; Constantinescu & Negrean, 1997).
Downy mildew is found most commonly in cool, humid production areas in New Zealand, North America, and Nordic countries, although in California, severe outbreaks have occurred in the arid interior San Joaquin Valley after spring rainfall. The disease is most prevalent during wet weather at temperatures of 18—22°C (Gubler & Rebollar-Alviter, 2017).
The pathogen overwinters as mycelium inside roots, crowns, and canes. When sucker growth begins in spring, P. sparsa closely follows apical shoot growth, infecting new stems and emerging leaves. Infected leaves on primocanes become the initial sites for sporulation, but leaves on infected fruiting laterals may also produce inoculum. Systemic leaf infection requires wet or humid conditions when leaves are unfolding. Sporulation is usually found in dense foliage near the cane or at ground level, where the humidity is highest. Weed growth and dense canopies favor the production of diseased suckers and fruiting laterals (Gubler & Rebollar-Alviter, 2017).
Airborne spores are produced during cool, wet nights and are disseminated by wind to foliage, including that of new primocanes, blossoms, and developing berries. The rapid infection of newly weaned plants from micropropagation in the United Kingdom was probably initiated by airborne inoculum rather than by systemically infected stocks (Gubler & Rebollar-Alviter, 2017; Williamson et al., 1989). However, the Fruit SEWG noted that wind blown transport was mostly considered effective over short distances. Resting spores can survive years, though the mechanism of infection from soil back into a growing plant is unclear (AHDB).
The disease can be dispersed by infected propagation material: Historical spread was considered to be with rose propagation material (CABI, 2021). The propagation of systemically infected plants has resulted in a disease incidence of 100% in newly planted fields in the United States and Mexico stocks (Gubler & Rebollar-Alviter, 2017). In New Zealand, systemic infection of Rubus propagation plants is considered to lead to disease in young plants (Herath Mudiyanselage et al., 2019), suggesting that infection of plants for planting is important for dissemination.
In New Zealand, oospores develop in leaves and sepals in the field in early summer, and they have been reported in leaves of naturally infected tummelberry in England. Their importance in the disease cycle is unknown. The presence in infected leaves may provide an inoculum source in propagation houses. Oospores formed abundantly in petals of tummelberry and in leaves of a red raspberry selection during inoculation experiments in vitro. The presence of oospores in field-grown crops raises the risk of new pathotypes arising and fungicides becoming ineffective (Gubler & Rebollar-Alviter, 2017).
There is no information on transmission of Peronospora sparsa through seed.
[In the responses to the questionnaire, NL commented that the pest was 'airborne and widespread in nature'.]
While the pathogen can spread via wind, the Fruit SEWG was uncertain on how widespread the pest was in EU countries. It was also noted that movement of propagation material is reported as an important pathway. Consequently, there was uncertainty whether plants for planting should be considered as a significant pathway compared to natural spread.

Yes

Much of the available literature on Rubus appears to be from Scandinavia, where in the mid 1990s P. sparsa affected arctic bramble crops severely, leading to quite a number of papers on control etc. as these berries are a valuable, if niche, crop.
Data on impacts on Rubus from other parts of the range tend to be harder to come by, though it is not clear if it is because everyone lives with the disease and it is not worth research and publication or if the disease is minimal e.g., controlled by routine crop husbandry and management to acceptable levels. As well as Rubus, it’s a known pest of Rosa spp.

Since 1994, farmers in Finland have reported serious yield losses in Rubus arcticus due to berries drying in the middle of the growing season. The yield loss is often 50%, but in some cases no yield is obtained (Lindqvist et al., 1998).
Koponen et al., (2005) note that disease is most severe in cool and rainy summers in Finland. ADHB state that: “Downy mildew, caused by Peronospora sparsa, is a major disease of blackberry. If left uncontrolled, it can cause crop losses of up to 50% and, in extreme cases, can result in complete crop loss in the UK, amounting to losses of £35,000/ha.”
In northern Scandinavia, P. sparsa causes demonstrable impacts in arctic bramble: “Cultivation of arctic bramble has decreased dramatically in Finland in 1990s following the devastating yield losses due to the ‘dryberry’ disease associated with Peronospora sparsa. In rainy summers the dryberry disease can cause a total yield loss in many cultivated Rubus species” (Kostamo et al. and cited references therein, 2015).
Fruit impacts: “The dryberry disease has been described in blackberry (Rubus fruticosus) as shriveling, hardening and splitting of the developing fruit if infected while still green, and as a loss of sheen in ripe fruits (O’Neill et al., 2002), and as a sudden drying of the fruit after infection” (Kostamo et al. and cited references therein, 2015).
Biotic and abiotic factors do not seem to have an effect on whether the pathogen is present or not: “occurrence of P. sparsa was not restricted to any particular type of a habitat. Infected plants were found both in sunny and shaded habitats, on plants growing in sandy, silty and organic soils, on seashores and stream banks, along roads, in abandoned fields and in swampy forests, and regardless of whether R.a. arcticus was scarce or abundant in the inspected habitat.” (Koponen et al., 2005). Disease incidence appears to be greater in crops than in wild Rubus: “Disease symptoms were not common in the plants of R.a. arcticus growing in natural habitats in Finland and Sweden, even though many examined populations contained infected plants. Hence, wild R.a. arcticus appears to withstand P. sparsa better than the cultivated clones. The conditions in the plantations (e.g., higher levels of nutrients in soil, cultivation in monoculture, no natural plant barriers preventing the spread of conidia, etc.) probably favour the pathogen.” (Koponen et al., 2005).

Medium

No

Candidate

Significant yield losses have been reported from Scandinavia, though little evidence has been provided from elsewhere.

Candidate

According to Gubler & Rebollar-Alviter (2017)
• Pathogen free material
• Select a site with no history of downy mildew
• Alternate hosts in close proximity should be avoided (like rose or blackberry)
• Removal of suckers and weed control to reduce humidity
• Old fruited canes should be removed and destroyed immediately after harvest to reduce build-up of inoculum.
• If grown in tunnels: adequate ventilation to reduce high humidity. Remark: The AHDB has published a review into control measures for P. sparsa on blackberry, which include good ventilation, reducing leaf wetness and humidity. Earlier work by O’Neill et al. (2002) in Suffolk found much lower levels of mildew on plants with sub-irrigation than on plants grown with overhead irrigation.
• Better not to use fungicides in nursery stock, to reduce the risk of selection of a resistant downy mildew strain to these fungicides. Remark: In Finland, “Development of the disease can be delayed but not prevented by treatment of plants with fungicides” (Koponen et al., 2005). Work in Suffolk by O’Neill et al. (2002) suggested that on a susceptible cultivar grown in conditions favourable for mildew development, fungicide regimes provided “some control”. Other authors find fungicides more effective: “The present study shows that elicitors of plant defence can effectively control downy mildew in arctic bramble in the field. The study corroborates the findings obtained under greenhouse conditions (Hukkanenet al., 2008). Phosfik and the fungicide Aliette 80 WG were equally effective in controlling downy mildew.” (Kostamo et al., 2015). CABI also consider that fungicides across several different functional groups deliver control, though there is the risk of resistance developing if the type of fungicide is not rotated.

Additional information:
• Experimentally, heat treatment alone or in combination with fungicides reduced systemic infection of Rubus sp. with P. sparsa in New Zealand (Herath Mudiyanselage et al., 2019), potentially allowing clean propagating material to be obtained.
• Different cultivars or species may show different susceptibility: “Differences in resistance to P. sparsa between R.a. arcticus and R.a. stellarcticus were indicated by the significant difference in symptom severity between the infected plants of the cultivars (clones) grown side by side in several Finnish plantations. In cool and rainy summers, such as 1998, several cultivations of R.a. arcticus were totally devastated by P. sparsa, shoots being killed by the middle of July. In contrast, the infected plants of cultivars of R.a. stellarcticus in the same plantations developed only foliar symptoms of modest severity and yielded satisfactorily. No significant damage caused by P. sparsa has been observed in the cultivations of R.a. stellarcticus in northern Sweden, even though this study shows that P. sparsa is widely distributed in wild R.a. arcticus and cultivated R.a. stellarcticus there. Therefore, the Alaskan R. arcticus subsp. stellatus may be a potential source of resistance to P. sparsa.” (Koponen et al., 2000). In the UK, “While some of the most popular blackberry varieties, such as Loch Ness and Black Butte, are very susceptible to P. sparsa, others, such as Karaka Black and Navaho, show some levels of resistance to the pathogen. The cultivar Chester Thornless is reported to have good disease resistance and grows well in the UK, but it is not prominent in commercial production. Globally, no commercial varieties are known to be fully resistant to P. sparsa, with other diseases and factors taking priority in current breeding programmes in the UK, such as fruit quality.” (AHDB, 2020)

Candidate