HVG Germany: Hopfen


Friends and Enemies


1. Downy mildew (pseudo peronospora humuli):

The most important fungal disease in hops. It was brought in from the USA to Europe via England. Downy mildew is spread by fungal spores. The infected parts of the leaves first take on a lighter colour. Grey fungal colonies develop on the undersides of the leaves. The flowers harden and do not develop, the cones show brown patches. The downy mildew warning service in the respective production areas facilitates well-targeted control of the disease. By estimating the flight of the spores, the moisture of the leaves and the saturation deficit of the air a warning is given daily on the risk of potential infection. The downy mildew is caused by the fungus pseudo peronospora humuli (Miyabe and Takahashi) Wilson belonging to the class of phycomycetes (= algal fungi). This fungus is an obligate parasite, i.e. it can only reproduce in the living hop plant from which it obtains its nourishment. In the same way as they occur, after secondary infection the fungal colonies are irregularly spread over the entire underside of the leaves. Unlike the primary infection the laterals, flowers and cones are also attacked by the secondary infection. New necks of the roots are infected by the fungus spreading downwards from the shoots which show secondary infection. Therefore the foundation is created for a primary infection of this plant in the coming spring.

2. Powdery mildew (sphaerotheca humuli)

pfl_krank_mehlt_2Powdery mildew occurs quite frequently in plants (roses, stone fruit ...). As a rule attacks become apparent by the whitish deposit on the upper surface of the leaves. The flowers and cones are deformed.
The fungi which cause the powdery mildew are obligate parasites on tall plants and are found all over the world; due to their appearance en masse they cause considerable economic losses. All green parts of the plants can be affected, in which the whitish floury patches more often form on the upper surfaces of the leaves than on the undersides of the leaves. Some of the mildew fungi form another ("secondary") mycel in an advanced stage of attack, which can be very extreme especially in the case of sphaerotheca and which consists of thick-walled, brownish hyphae. Sometimes mildew on grass also develops a reddish brown, feltlike mycel deposit.



3. Botrytis (Botrytis cinerea)

krankheiten_7The botrytis fungus occurs more often in years with frequent rainfall during the burr. Only the cone is infected. Individual leaves or the tips of the cones turn reddish brown.

Again and again botrytis cinerea is classed as a weak parasite because in most cases it needs a host plant which is weakened or damaged before infection occurs. Above all inadequate cultivation conditions, such as when hops stand too close together, lack of light and air, too much moisture in the air and soil, greatly fluctuating temperatures, damage through frost, scorching or attacks by insects encourage the infestation.
The fungus forms black sclerotia, with which it can survive unfavourable conditions and with appropriate conditions from which conidiophores develop again. The brownish areas of infestation can form on stems, leaves, flowers (botrytis blight) and on young sprigs but also roots and the necks of roots can be infested.

4. Wilt in hops (verticillium albo-atrum, v. dahliae)
pfl_krank_welk_1pfl_krank_welk_2The wilt disease on the part of the plant above the ground appear (according to the weather conditions and the prevailing "stress situations") between the burr and cone development up until harvesting. First of all the bottom leaves show signs of turning yellow. Later on brown patches become progressively visible finally resulting in the leaves dropping off.

One or all of the trained bines in one hill can die off either one after the other or simultaneously. A definite symptom for an attack by the verticillium fungus, which lives in the soil, is when the xylem turns brown. It is possible that the following year the new shoots of an already diseased plant show no symptoms. Nevertheless attacks spread over several years weaken the rootstock so that in the spring this may result in the shoots developing later and to a lesser extent.

Chemical control is not possible. Moderate fertilization (with nitrogen etc.), loosening the soil and avoiding damp ground reduce the susceptibility of the plant for wilt. The following varieties are resistant to wilt disease: Northern Brewer, Hüller Bitterer, Perle and Orion.