Downy mildew in Ontario hops

There have been many reports of downy mildew in Ontario hop yards in the last few weeks, likely related to the wet weather experienced in the province for much of the spring. Downy mildew is one of the most serious diseases of hops in many regions of the world, and was partly responsible for the collapse of the hops industry in eastern North America in the early part of the 20th century. Downy mildew affects both yield and quality of cones, and sufficient infection can cause total crop loss or even plant death. The hops downy mildew pathogen, P. humuli, is specific to hops and is quite distinct from downy mildew diseases of other crops. However, recent evidence suggests that this pathogen is closely related to P. cubensis, the causal agent of cucurbit downy mildew.

Identification

Hops downy mildew produces characteristic, systemically infected shoots which are called "spikes". Spikes are stunted, with shortened internodes (the length of stem between points where leaves attach) and chlorotic, yellow leaves that often curl downwards and may appear brittle (Figure 1). The undersides of infected leaves are often covered with dark, purple-black spores (sporangia) (Figure 2). Leaves may become necrotic and die, with necrosis beginning at the base of the shoot and progressing towards the tip.

Figure 1. Primary or basal spikes produced by a systemically infected hops plant in the spring

Figure 1. Primary or basal spikes produced by a systemically infected hops plant in the spring

Figure 2. Sporulation on the underside of a basal spike.

Figure 2. Sporulation on the underside of a basal spike.

Spikes may emerge from various points on the hops plant. Infected shoots arising directly from the crown beginning in early spring are called basal, or primary, spikes (Figure 1). Those that arise from infected apical meristems (growing points), either lateral shoots or trained bines, are called secondary, or aerial spikes (Figure 3). Because basal spikes arise from infected buds, symptoms occur from the ground up. Symptoms on secondary spikes will arise only from the point of infection outwards, with plant tissue beneath that point appearing normal. Internodes on aerial spikes may also not be as shortened as on basal spikes. Infected branches may desiccate, and trained bines may stop developing and fall off the string after infection.

Figure 3. Systemically infected lateral shoot (sometimes called an aerial spike).

Figure 3. Systemically infected lateral shoot (sometimes called an aerial spike).

Downy mildew spores landing on and infecting leaves will produce localised lesions. These first appear on the upper surface of the leaf as yellow, water-soaked lesions that are often confined by the leaf veins and take on an angular appearance (Figure 4). As the infection progresses, the lesions become light to dark brown and die, and are surrounded by a yellow halo. Lesions often develop a purplish grey to black growth of sporangia on the lower leaf surface. This growth is most noticeable in the morning when leaf surfaces are wet, and may not be as visible when conditions are very hot and dry.

Figure 4. Yellow lesions confined to the leaf veins on the upper surface of the leaf.

Figure 4. Yellow lesions confined to the leaf veins on the upper surface of the leaf.

The disease can also spread to inflorescences, which can become dark brown and desiccated and may fall off the bine. Infected cones appear hard and brown, and may not develop properly if infection occurs early in the season. Late season infections may only affect bracts, resulting in stripes of brown tissue on cones. Purplish brown sporangia may also appear under affected bracts.

The appearance of infected crowns varies with cultivar from apparently healthy to various stages of rot. Infected crown tissue may have reddish-brown to black flecks or streaks in the white crown tissue near the bark; however, note that some cultivars have reddish tissue in the center of healthy crowns.

Downy mildew can sometimes be confused with frost damage or other fungal diseases. Frost damage can cause stunting, chlorosis and necrosis of leaves and shoots that may appear somewhat similar to downy mildew, however frost-damaged leaves may also develop a silver appearance and will not have purplish to black sporulation on the lower leaf surface (Figure 5). Recent weather conditions will also be a consideration in diagnosing frost damage. Powdery mildew produces white circular powdery fungal growth on the upper surface of leaves, while downy mildew lesions are yellow to brown on the upper leaf surface. Some other fungi, such as Phoma, will occasionally infect hops and can cause brown lesions on the leaf surface; however these are generally not defined by the leaf veins and will not have the purplish to black sporulation on the leaf underside.

Figure 5. Frost damage (shown here) can look very similar to downy mildew damage.

Figure 5. Frost damage (shown here) can look very similar to downy mildew damage.

Biology

Downy mildew overwinters as mycelium in dormant hop buds or crowns. Mycelium can spread into buds, leading to the development of basal spikes in the spring. Shoots arising from infected crowns are not always infected, and an infected crown may produce no basal spikes, or a mix of basal spikes and healthy shoots. When weather conditions are favourable, mycelium within infected shoots may produce sporangia on leaf undersides at night, which appear as a purple to black growth (Figure 2). The spores are released in mid-morning to early afternoon, particularly during rain events, and spread to leaves or other plant tissue. There, they release zoospores that swim on films of water on leaf surfaces and enter the plant through the stomata.

As long as moisture is present, downy mildew zoospores can infect hop leaves, buds, cones or stems. Leaf infections tend to be localized, producing more spores to infect other tissues; however infections of apical meristems may become systemic, producing secondary shoots and/or progressing through the shoots towards the crown. Crown infection reduces carbohydrate reserves and can weaken or kill plants.

Hops downy mildew is favoured by wet conditions and moderate to warm temperatures. High humidity (greater than 80% and nightly temperatures above 5 °C) are required for sporulation. Moisture on leaf surfaces is required for infection to occur, with the duration of leaf wetness required for infection varying with temperature. For example, infection of leaf surfaces can occur when surfaces remain wet for 1.5-2 hours at 15-29 °C, but at 5 °C, infection requires 24 hours or more of leaf wetness. Downy mildew leaf and shoot infection is most likely during warm temperatures when plant tissue remains wet for 4-8 hours.

Management

Successful management of hops downy mildew requires an integrated approach, as no single tactic is sufficient to manage this disease. This is true for both conventional and organic production systems.

Hops downy mildew management should begin at planting. Ensure that cuttings or rhizomes are free of downy mildew at planting, if possible, by purchasing plant material from reputable and experienced suppliers. Choose resistant varieties when possible. Although no hops cultivar is totally immune to downy mildew, some are more susceptible to the disease than others. Cascade, Fuggle, Magnum, Newport, Perle and Willamette are generally considered to be resistant in the Western US while Centennial, Cluster, Galena, Glacier, Mt Hood, Northern Brewer and Nugget are all reported to be susceptible. In some trials in eastern North America, however, Nugget was found to be more resistant, and Newport more susceptible. These differences may be due to the presence of different strains of the disease in the two growing regions. Variety trials currently being conducted at the Simcoe research station are monitoring for resistance to downy mildew and other pests.

Sanitation practices aimed at removing pathogen inoculum and promoting air flow are also important in managing downy mildew. All basal foliage should be removed from the hop yard in late winter or early spring. In general, the more thorough the pruning and the later the pruning, the greater the reduction in levels of disease, however note that late pruning can also reduce yield in some varieties. Growers removing infected basal spikes during training should remove infected material from the hops yard and burn or bury it, as spores on plant tissue can be blown back onto healthy plants if left nearby.

Once trained bines have reached 2 m in height, remove or strip the lower 1 m of leaves and lateral branches to encourage airflow, reducing humidity and making the environment less favourable to the pathogen. It also limits the ability of the pathogen to spread into the upper canopy. In Ontario, stripping must be done e there are no chemicals registered for this purpose on hops in Canada. Take care in selecting the date and severity of stripping, as it can reduce carbohydrate reserves in the roots by limiting photosynthesis, affecting yield in subsequent seasons.

Other practices that can help manage downy mildew include ensuring nitrogen fertilization is limited to what is required by the plant, as excess nitrogen can promote disease. Avoid overhead irrigation, as this increases the period of time that leaves remain wet.

Preventative chemical sprays are often required when environmental conditions are favourable to disease development. Remember that most products registered for hops downy mildew are preventative rather than curative, which means they must be on plants prior to infection. This is why it's so important to monitor weather and apply products prior to periods of prolonged leaf wetness. Additionally, these will not provide season long protection, particularly under high disease pressure situations, so growers should be prepared to reapply these products in rotation throughout the season. When selecting fungicide rotations for hops downy mildew, it is important to consider the product's fungicide group (for resistance management), the re-entry and post harvest interval and the number of applications permitted per season. For detailed information on registered products and rotations, refer to the ONspecialtycrops blog at www.onspecialtycrops.wordpress.com.


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