All of our hop plants are certified disease free, but they may still be susceptible to pests and diseases that exist in the air or soil in your area. The old adage ‘prevention is better than cure’ will usually help you to keep your plants happy and healthy, but should you have any problems you will find the solution below to the usual suspects.
Is my hop plant resistant?
No variety of hops will be completely resistant to any of the pests or diseases we discuss below, although some varieties have been developed to offer some degree of resistance, or less susceptibility. We have briefly outlined this information in the product descriptions for each hop variety, and we would suggest doing a bit more reading and research if you are concerned about the susceptibility of your plant.
General Advice for healthy hops
- Give your hops a bit of room. If you position your hops close to other plants you will find that diseases may spread more easily and quickly. Aim for at least 2 metres between plants.
- Improve air circulation around plants.
- Many of the diseases below can be caused by wet conditions. Water the base of the plant in the morning to allow the plant to dry out during the day.
- Clean up leaf litter from the base of your plants.
- Prevention is better than cure but issues that do arise can be treated locally.
The main pests and diseases – how to identify and treat them
Downy mildew, also called ‘spike’, is a disease of the foliage, caused by a fungus-like parasite that spreads by airborne spores. This disease spreads among wet leaves and in damp conditions.
The first signs you might have an issue is the appearance on leaves of spores – light green/yellow “oil spots” (so called due to their greasy appearance).
To prevent downy mildew remove the damp conditions it enjoys. If you water your plants, do so in the morning so the plant can dry during the day. Improve air circulation. Remove and carefully destroy any parts of the plant showing infection.
Organic copper sprays are available to treat an outbreak and can be applied according to the instructions.
Powdery mildew, also called ‘mould’, is a fungal infection that typically starts on the underside of leaves, on stems, and on buds, before showing as white powdery spots on the top side of leaves, especially on low leaves.
The fungi spores can overwinter in plant debris and are transported by wind and insects. Although unattractive, and may stress your hops, it is unlikely to be fatal.
Remove and destroy infected leaves – do not add the leaves to your compost. Wash your hands and clean your garden tools with alcohol wipes to prevent further spread.
Fungicides containing potassium bicarbonate, neem oil, or sulphur can be used to treat outbreaks.
Verticillium wilt is caused by soil-borne fungi, infecting plants through their roots and then growing upwards through the xylem, causing wilting throughout the growing season. Verticillium wilt is most prevalent in nitrogen rich soils.
The first signs will be the yellowing and shrivelling of lower leaves and the sudden wilting of part or all of your plant. Swollen and discoloured bines will help to distinguish this from other diseases.
If you suspect your plant might be suffering from Verticillium wilt, firstly take care not to spread infected soil to other plants in your garden. Clean your tools and your boots.
There are no chemicals available to treat Verticillium wilt and the fungi can reside in the soil for many years.
Hop Mosaic Virus
Hop Mosaic Virus is carried to your plant by aphids. It can be difficult to detect HpMV as symptoms look similar to many nutrient deficiencies. Many hop varieties will not show symptoms, however Chinook and Goldings can be sensitive to it.
Symptoms to look for are yellow, white or green stripes or spots on foliage, or curling of small leaves. Growth of your hops may be stunted with less cones and the plants can survive for several years with the virus.
Hop Mosaic virus can over winter on perennial weeds and infection can exist in the soil, seeds or containers.
There is no cure so every effort should be made in prevention – clearing away weeds, using clean tools, and if you think you have an infection, remove and destroy your plant.
Red Spider Mite
If your hop plant has Red Spider Mites it will start to look unhealthy, with a dusty appearance on the underside of the leaves. The mites are not visible to the naked eye, but may be seen with a magnifying glass. Our farmer has a clever trick to confirm if this is indeed the issue facing your hop plant – take a pinch of dry soil and sprinkle it on to the underside of the leaves of your hop plant. The fine dust will stick to and reveal the webbing of the mite.
Red spider mite can also affect azaleas and camellias, so take care if your hops are planted near to these plants.
You can discourage red spider mites before they get started by keeping the area around your hop plant clear and to water them during dry periods as red spider mites prefer dry environments.
If you think you have an issue with red spider mites you can control them by introducing their natural predators, such as the predatory mites amblyseius andersoni or amblyseius cucumeris.
If you prefer you can use pesticides – insecticidal soaps and oils work best – however the disadvantage of this method is pesticides will also kill the natural predators you want to encourage, and the red spider mites may simply move from the treated area to non-treated areas.
Hop Damson Aphid
Also known as Phorodon humuli apterae, or commonly as ‘Green Fly’, are 1-2mm in size, with a shiny white to pale yellow-green colour and three dark green stripes on their abdomen.
They are visible from spring to autumn, when daytime temperatures exceed 15°C, and continue through the growing season. They can cause major damage by removing nutrients and moisture from plants. They also secrete a sugary honey dew frass, the perfect growth medium for sooty mould (see below).
Damaged leaves may curl and wilt, while heavy infestations can cause defoliation. Cone feeding can cause wilt like symptoms in the cones and browning.
Damson hop aphids overwinter as eggs on Prunus species (plum, cherry) and in May winged females are produced and travel to hop plants where as many as 10 generations may occur in a season.
Aphids can be controlled by introducing more of their natural predators, like lacewings and ladybirds, or by using a spray of dilute soapy water.