Disease: false meldew

False mildew on hops Pseudoperonospora humuli (MIY. et TAK. ) WILSON)

Peronospora is caused by the fungus Pseudoperonospora humuli (Miyabe and Takahashi) Wilson belonging to the class of phycomycetes (= algae fungi). This fungus is an obligatory parasite, i. e. it can only multiply in the living hop plant from which it draws its food.

This fungal disease occurs each year in different degrees. All parts of the plant can be infested, infestation of flowers and cones can lead to a complete loss of yield. A distinction is made between primary and secondary infection.

Classification: Mastigomycotina, class Oomycetes, order Peronosporales, family Peronosporaaceae.

Primary infection:
The primary infection is caused by the fungus that has overwintered in underground plant parts and penetrates into the young shoots in spring. Affected ground shoots are compressed and coloured yellow-green ("bubiköpfe"), top and side shoots do not grow any further and dry up. At the leaf undersides of such shoots, a greyish-black coating of fungal spores often forms along the leaf veins. The fungus usually overwinter as mycelium (fungal threads) in the root necks of the hop plants. Rhizomes and roots may also be affected. The infected tissue is often reddish-brown and may die after a few years. In spring, the fungus penetrates from the root neck or underground plant parts into the young shoots, causing the first or primary infection.

Another wintering form of the fungus, which can also cause a primary infection, are the oospores. However, the primary infection is caused by oospores much less often than by wintering mycelium. The oospores are formed within the vines and leaves of the diseased hop plants towards the end of the growing season of the hops. They reach the ground with falling leaves and other plant remains.

In the coming spring, the oospores germinate and the swarm or zoospores that become free may infect the sprouting hop shoot. The primary infected soil shoots are strongly compressed and yellowish discoloured. The fungus penetrates from the primary infected shoots further into the bat stalks and along the leaf veins into the leaves.
Primary infection of Peronospora in hops
The fungal mycelium grows in the spaces between the plant cells, the so-called intercellulars. From there, it drives haustorias (continuations) into the interior of the surrounding cells, from which the fungus feeds. These cells are damaged in such a way that they begin to die, causing the discoloration. When night temperatures are above 7°C and a relative humidity of 50% is exceeded, a dense grey-black coating forms on the underside of the leaves near the leaf veins. This fungal grass is formed when the fungus penetrates into the air space via the crevices at the underside of the leaf with reproductive hyphae (the conidium carriers). Under the microscope you can see that the reproductive hyphae are tree-like and carry oval structures, the zoosporangia, at their ends.

Fig. 1: False mildew primary infection (so-called "Bubikopf")

Secondary infection:
Secondary infection is caused by the spores that get to the plant by wind and rain. Yellowish spots form on the top of the leaf, which later turn brown. There grows on the underside the grey-black fungal spore coating. Infected flowers die, harden and the formation of bulbs is prevented. In the case of an incipient cone infestation, the front leaves are more discoloured than the cover leaves, which leads to a checked appearance of the cone. In the final stage the whole cone is reddish-brown. The zoosporangia easily detach from the conidium carriers and are transferred by the wind to other plant parts and hitherto healthy plants. There, when dripable liquid water in the form of rain or dew is present, the zoosporangia release the floating zoospores (also called swarm or summer spores). With the help of a germ tube, these penetrate the plant via lens cells and cleft openings, which is then re-infected (secondary infection). Once the fungus has entered the plant in this way, the disease of these parts of the plant can no longer be prevented by plant protection products. A few days after the infection, the symptoms of the disease appear: yellowish-coloured shoots, especially against the tip, so that the leaves sit tightly together ("bubic heads");
("Bubicheads"); the leaves show brown spots. With appropriate weather conditions, new conidia carriers with zoosporangia form from there and thus the further spread of the disease. The fruiting time, i. e. the time from infection to the emergence of conidium carriers with zoosporangia, varies in length depending on the prevailing temperature and humidity conditions. Optimal for the formation of zoosporangias is a relative humidity of 90-100%; below 60% relative humidity no more zoosporangias arise. The most favorable temperature for the formation of zoosporangia is between 20 and 22° C; below 5° C no more zoosporangia are formed. According to Zattler (1931) the fruiting time of the variety Hallertauer mfr. About four days. At an average temperature of only 11°C, it extends to 11 days. The knowledge of the favorable conditions for fructification makes it understandable why the Peronospora appears so strongly in rainy summers with many humid days.

Fig. 2: False mildew secondary infection

Flower infestation According to their formation, the fungal grasses formed after secondary infection are distributed irregularly over the entire leaf underside. In contrast to primary infection, secondary infection also affects side shoots, flowers and umbels. The secondary affected side shoots are often only yellowish and compressed at the shoots and only there are the light- to dark-brown discoloured leaves, while the shoots grow normally at the base. The latter cannot occur in the case of primary infection, as the fungal mycelium grows from the base into the shoots. Affected flowers turn brown and may dry out and fall off.

New root necks are infected by the downward growth of the fungus from the secondarily infested shoots, thus creating the basis for a primary infection of this plant in the coming spring.

(After: Wittmann, Wolfgang, Atlas of Ornamental Plant Diseases, Blackwell Wissenschaftsverlag Berlin, 1995)