Impact of infection timing and autumnal fungicide applications on perennation of pseudoperonospora humuli and severity of hop downy mildew

Authors: RICHARDSON, BRIANA - Oregon State University; Gent, David - Dave

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/11/2023
Publication Date: 4/20/2023
Citation: Richardson, B.J., Gent, D.H. 2023. Impact of infection timing and autumnal fungicide applications on perennation of pseudoperonospora humuli and severity of hop downy mildew. Plant Disease. https://doi.org/10.1094/PDIS-02-23-0268-RE.
DOI: https://doi.org/10.1094/PDIS-02-23-0268-RE

Interpretive Summary: Downy mildew is a potentially devastating disease of hop that limits productivity, sustainability, and where certain hop cultivars can be produced. The organism that causes hop downy mildew survives the winter dormant season as a systemic infection in infected hop rootstocks. In this research, we sought to understand how infection of hop plants in one year influences the level of the disease that develops in the next year. We found that infection over a broad period of time from early summer to early autumn may allow the downy mildew organism to overwinter successfully. In spring, the disease emerged in sync with the development of health hop shoots, emerging over a period of greater than three months. In general, infection of plants in mid-summer resulted in the most severe disease in the next year, whereas delaying infection in autumn tended to reduce overwintering. We found that fungicide applications made in autumn had little benefit on disease suppression in the next year. Overall, this research sheds new understanding on ecology of the disease and will help to target control measures to the periods of greatest impact.

Technical Abstract: Pseudoperonospora humuli, causal agent of hop downy mildew, is known to overwinter systemically in the crown and developing buds of hop (Humulus lupulus). Field studies were conducted over three growing seasons to quantify the association of infection timing to overwintering of P. humuli and development of downy mildew. Potted plants were inoculated sequentially from early summer to autumn, overwintered, and then evaluated for symptoms of systemic downy mildew in emerging shoots. Shoots with systemic P. humuli developed after inoculation at any time in the previous year, with the most severe disease typically resulting from inoculation in August. Independent of the timing of inoculation, diseased shoots emerged coincident with emergence of healthy shoots, beginning as early as late February and continuing through late May to early June. Surface crown buds on inoculated plants had internal necrosis associated with P. humuli at rates ranging from 0.3 to 1.2%, whereas P. humuli was detected by PCR on 7.8 to 17.0% of asymptomatic buds depending on the timing of inoculation and year. Four experiments were conducted to quantify the impact of foliar fungicides applied in autumn on downy mildew the following spring. There was a small reduction of disease in only one study. Together, these studies indicate that infection by P. humuli that leads to overwintering can occur over a broad period of time, but delaying infection until autumn tends to reduce disease levels in the following year. Given that foliar fungicides applied in autumn had minimal effect on the severity of downy mildew in the next year, in established plantings, control efforts for the disease appear most critical during spring and summer months when downy mildew tends to be most active and damaging.