Downy mildew of cocksfoot: Temperature-dependent patterns in sporulation and zoospore release

Abstract

Cocksfoot (Dactylis glomerata) is a key pasture species in New Zealand. Since 2018, a downy mildew disease has been observed affecting seed production in the Canterbury region. Symptoms initially appear as irregular yellow to brown lesions on leaves and banding on stems, which progressively expand, leading to chlorosis, seed head bleaching, and ultimately reduced seed yield and quality. The causal agent has tentatively been identified as the obligate biotrophic oomycete, Sclerophthora cryophila, pending further phylogenetic analysis. This study aimed to assess how temperature influences the sporangia formation, sporulation and zoospore behaviour of this pathogen. Infected plant tissues were floated on water to induce sporulation and incubated at temperatures ranging from 4°C to 24°C. Sporangia retained on tissues were collected using Sellotape; free zoospores and sporangia in water were counted using a haemocytometer, with swimming speed being used to score zoospore activity. The results showed that lower temperatures, particularly 4°C, enhanced sporangia development and sustained sporulation, suggesting the pathogen is well-adapted to cool climates. Zoospore release peaked at 14°C, where rapid encystment was also observed. Additionally, higher temperatures (19°C－24°C) increased sporangia detachment from tissues. These findings show the role of temperature in the cocksfoot downy mildew life cycle and suggest environmental conditions may drive disease outbreaks in cocksfoot, informing future epidemiological studies and management strategies