Hops has been compared to gambling due to the difficult predictability in growing the plant, due to the various disease and fungus that can cripple fields and ruin the yields. The range of problems that can arise in growing the plant include bacterial diseases, fungus and mildew, virus diseases, as well as pest and parasitic invasion. In fact entire crops can be quarantined and burned due to problems. More over these diseases can vary from year to year providing a large amount of risk to the growers.
Downy mildew, caused by Pseudoperonospora humuli fungus, is one of the most prevalent. This effects the shoots of the plant is and visible in the early growing season. The shoots are will become brittle and the growth is severely effected. This symptom is passed on to the leaves which will appear wilted and deformed. Hops growing in the United States was originally started on the east coast, and this disease was a primary motivator to move the crops to the Pacific Northwest where the mildew does not persist. The fungal spores appear in black and gray masses on the underside of leaves and propagate in the wind. The disease is associated with warm and wet weather, and can completely fill the plant from the roots, or in milder cases effect the formed cones leaving them useless to brewers. This disease is both an enemy to the hops grower as well as grape growers and vineyards.
Verticillium wilt is another very serious and common disease that can cripple hops, and force entire fields to be destroyed. Conidial spores of the fungus enter through the roots and cause damage to the vascular system.
This fungus is also uncommon in the Pacific Northwest, and that draws attention to the reasons why that area has a lot going for it for hops production. This is an exception when the plants or soil is treated with insecticide heptachlor, which can leave the roots more vulnerable. This disease can effect a large number of plant varieties including trees, shrubs, and vegetables. This disease was discovered during the early 1900's and is described to have 2 seperate strains including progressive and mild. The strains are only distinguishable by the reactions of the plants. Hop varieties are grown specifically for their resistance, while others such as Wye Target are bred in ways that reduces their vulnerability.
Powdery mildew known as hops mould is know for growing in warm and dry conditions. Its effects used to ravage crops, but has largely been eradicated with the use of sulfer. For the home grower garlic contains high amount of sulfur and can be used to as a preventative. Sphaerotheca humuli, as its known, spreads across leaves and cones and appear as white or grey pustules. The mildew will appear as red spots as it overwinters and will stunt the growth of the leaves and buds as nutrients are removed. Powdery mildew can spread rapidly over several days but can take close to a week before the symptoms can readily be seen.
Other Fungal Diseases
To help put it into perspective the list of fungal diseases that can cause harm is not a short one. Below are some that have been reported:
- Alternaria blight and cone disorder - Common in warm and rainy weather Alternaria appear as small yellow to brown spots. The disease will eventually cause leaves to wilt and die.
- Anthracnose - Anthracnose is a generally not sever and causes blighting of the leaves turning them to a lighter yellow shade. It is more common in wetter areas west of the Cascades.
- Armillaria root rot (shoestring root rot) - Armillaria root rot has been reported in all areas of the United States and is generally found in warmer tropical climates. The disease causes growth decay.
- Ascochyta leaf spot - Common in hot and dryer areas, the fungus of Ascochyta leaf spot will form yellow to brown patches on the leaves.
- Black root rot - Thielaviopsis basicola fungus causes black root rot was first reported in the South-Eastern United States. The fungus can persist in soil for years and prefers cooler moist environments. It effects the root system and inhibits the plants growth. The roots will often turn from dark brown to black
- Canker - Canker is a term for a broad variety of fungal disease which is common in citrus and pine trees. It causes poor growth of the shoots and causes a significant reduction in the hop cones.
- Cercospora leaf spot - Cercospora beticola is the fungus that causes Cercospora leaf spot. High humidity or moisture on the leaves provide favorable conditions for the fungus. Infected leaves will eventually die.
- Cone early maturity - Late season infection of P. macularis will cause a smaller more condensed fruit.
- Cone tip blight - Cone tip blight can be caused by both Fusarium avenaceum and F. sambucinum fungus strains. Browning of the cone tip will begin near the cones maturity
- Cylindrosporium leaf spot - As the name implies, this fungus causes spots to develop on the leaves of the hop plant, and also effects trees. The spots are dark brown and circular in shape with a yellow halo around the edges.
- Gray mold - Gray mold is a common fungus infection in many plants and prefers humid conditions. The symptoms will appear as shoot blight and crown rot.
- Mycosphaerella leaf spot and cone disorder - Mycosphaerella leaf spot persists in wet weather, and affects both the leaves and cones of the hops plant. This disease also effects tree farms and has been combated by interspersing olive trees within the farm. The olive leaves cover the trees leaves on the ground in the fall, and control the spread of the disaease. Mycosphaerella appears as a gray mold.
- Phyllosticta leaf spot - Phyllosticta attacks leaves of hops and range from a few brown spots to engulfing the entire plant. It creates irregular brown to yellow lesions. At the center of the lesions will result in dead tissue that will eventually break off and leave a hole in the leaf. Moisture contributes to providing a good resting ground for Phyllosticta spores. Overhead irrigation should be avoided to keep moisture on the leaves to a minimum
- Red crown rot - Phacidiopycnis spores will create what is known as red crown rot by penetrating injured tissue. The fungus can survive in soil and loose debris, extending its potential for infecting nearby plants. Leaves will turn yellow and loose strength, the fungus also effects the bark and roots.
- Rosellinia root rot - Rosellinia root rot is also known as dematophora root rot and white root rot. The fungus can persist for years in dead or buried organic matter. It grows white mycelial patches which can easily be reproduced by containing the rotted matter in a moisture packed container. The mycelial will rapidly grow. The effect on hops is yellow foliage and weakened plant, not to mention its debilitating effect on its ability to produce flowers.
- Sclerotinia wilt - Sclerotinia effects many plant species and endures winters in debris and soil. It effects roots near the surface. It can spread by root contact to other plants, and call also produce fungus on the surface of which spores can spread by wind and water.
- Septoria leaf spot - Septoria leaf spot is common and tomatoes and debilitates the grwoth and production of foliage. It thrives in high humidity in dense places where airflow is at a minimum. Avoid moisture on the leaves. If a plant is infected, it should likely be destroyed to avoid spreading to others.