Hop downy mildew
The first appearance of the disease in the spring is the development of spindly, stunted shoots. They have short, fat internodes and yellowish down-curled leaves, which develop dark-brown or black masses of sporangia on their lower surface. These are known as 'primary basal spikes'. They are systemically infected, developed from mycelium which has overwintered in the rootstock. Infection of shoots via the stipules of the growing point leads to 'secondary basal spikes' with normal lower internodes with healthy leaves and typical symptoms on the upper part of the shoot. When a growing bine of 1.5-2.5 m or more is infected by downy mildew, the tip of the bine suddenly transforms into a spike ('terminal spike') and thereafter does not climb up the support. The majority of terminal spikes are caused by secondary infection and some could be caused by overwintered mycelium (Mori, 1966), which can be transported within the climbing shoot and colonizes the tip much later. Stunted lateral shoots ('lateral spikes') arise in response to infection of axillary buds and shoots.
Secondary infections may occur on leaves, stems, growing tips, burr and cones. On the leaves they are seen either as small spots limited by veins or larger more angular brown areas. Diseased burr hardens, turns brown and falls off. Later attacks on the cones turn the bracts and bracteols reddish-brown.
The downy mildew infection in hop roots, rhizomes and crowns is centred in the pith, phloem, xylem, parenchyma and cortex. A common symptom is a reddish-brown coloration of infected tissue.