Evaluation continues

Candidate

With respect to the primary infection, P. halstedii is a soil-borne pathogen. Its oospores serve as primary inoculum to underground tissues of young sunflower seedlings. It may also be windborne, causing secondary infection of leaves and/or inflorescence however the significance of windborne sporangia in disease initiation has long been regarded to be low. The fungus is also seedborne: the affected seeds carrying mycelium and/or oospores internally and seed tests are available to detect infection. Oospores develop mainly in root and lower stem tissues of mildewed plants, with or without visible symptoms and, with plant residues of the preceding sunflower crop, they come incorporated into the soil. Oospores are long-lived and are able to survive for at least 6-8 years. It is found that the fungus was present in a significant percentage of seeds from naturally infected plants and seeds gave rise to apparently healthy seedlings with no typical symptoms (latent type of infection) but the pathogen sporulated more often on the roots of these symptomless plants from infected seed. In dry years the number of pathogen-contaminated seeds is very low and may not exceed several in one thousand, but may be much higher after a cool and humid period. For example, in one case nearly 10% of seeds from a field in Germany were contaminated and another under favorable experimental conditions, observed fungal structures in 28% of the seeds examined (CABI, 2016).
Plants for planting are a host and plants grown for ornamental cut flower production from infected seed could be infected. Seed is a significant pathway, including for exchange of strains which overcome control measures (resistant varieties and metalaxyl fungicide treatments).

Yes

Information on impact for cut flower ornamental crops is limited, however the flower heads may be discoloured, show white downy growth or "hunched" bearing of the flowers (ANSES, 2013). When climate conditions are conducive, the potential impact in the absence of management measures is high: P. halstedii is capable of causing yield losses of up to 100% (total loss of contaminated plants). The majority, if not all, of systemically infected plants either die prematurely or hardly produce viable seed, they make no contribution to yield. Reduction in seed yield may also be due to pre- or post-emergence damping-off of severely mildewed seedlings under very favorable conditions. With suitable management measures, in particular the use of varieties resistant to pathogen populations, the economic impact can be estimated (on the basis of the French situation) to be, on average, a 3.5% yield loss for the PRA area (EU COM, 2016; ANSES, 2013; CABI, 2016). Other management measures are seed treatments, fungicides, rotation of crops and removal or ploughing in of infected debris etc.

Major

No

Candidate

Information of the impact on ornamental sunflowers is limited. When climate conditions are conducive, in the absence of management measures, the potential impact on sunflower for oil production is high. Impact on ornamental sunflowers can be extrapolated from that.

candidate

Visual examination, uprooting of all plants with symptoms from the infected field.