Downy Mildew is caused by the notorious plant pathogen, Pseudoperonospora humuli, an organism that is considered by biologists to be an oomycete protist. Pseudoperonospora results in reduced yield, poor hop quality, and, in severe cases, plant death. With an ever-increasing number of gardeners, home, and local brewers interested in growing their own hops, it is essential that they understand the cause and symptoms associated with this severe pathogen. If you grow hops, you should read this short article, then watch the video explaining the life cycle of downy mildew in hops plants
Symptoms Of Downy Mildew In Hops Plants
Growers begin to see signs of downy mildew in early spring. Pseudoperonospora affects hops plants grown in containers and hopyards. It is one of the most important diseases of hops that are grown in wet and humid regions. Mild temperatures (~65°F) and moisture resulting from rain, overhead irrigation, and morning dew are ideal conditions for infection. Microscopic flagellated zoospores are produced which swim on the surfaces of a leaf, entering through plant stomata and setting up an infection.
Leaves that are infected have black lesions while cones that are infected become brown, harden, and sometimes do not develop correctly. Pseudoperonospora continues to grow and invades the hop tissues, eventually killing healthy plants. The infection moves throughout the entire plant, including the bines, buds, and rhizomes. More zoospores are produced on the underside of leaves, which becomes blackened with masses of sporangia.
Video: Downy Mildew In Hops Plants
Click on the video and see the life cycle of Downy Mildew and how it infects hops plants.
A Change In Classification
For many years, biologists believed that oomycetes were true fungi. In fact, the “mycete” suffix is reserved for fungi. The basis for its classification was based on filamentous cells and the formation of sporangia, which are common characteristics of true fungi. With more advanced techniques available to biologists, there is now evidence that oomycetes are not related to fungi and are more closely associated with a group of protists called Stramenophila. So even though their placement on the tree of life has changed, their name has not.
What Makes Oomycetes Unique?
Oomycetes are unique organisms that differ metabolically, genetically, and in their cell structures. Pseudoperonospora is an obligate plant pathogen, meaning that it requires a living cell to complete its life cycle. Additionally, this organism produces motile zoospores with two flagella. One flagellum is whiplike, while the other is a ‘tinsel’ flagellum. This important characteristic is another reason why they are now classified as Stramenophila.
There are several differences between the characteristics of oomycetes and fungi. For example, the cell walls of Pseudoperonospora are composed of cellulose rather than chitin, and their cells typically do not have septations. Another difference is in the vegetative state that is composed of diploid nuclei, whereas fungi have haploid nuclei. Additionally, oomycetes and fungi have different metabolic pathways for synthesizing the amino acid, lysine, along with enzyme and mitochondrial differences.
Choosing Pseudoperonospora Resistant Varieties
Listed below are hops varieties that are moderately resistant and resistant to downy mildew.
|Hops Variety||Usage||Downy Mildew Susceptibility|
|U.S. Tettnanger||Aroma||Moderately Resistant|
Currently, growers manage downy mildew by removing basal foliage during spring pruning and frequent applications of fungicides. An extensive list for Disease Management and Control for both gardeners and large scale conventional growers can be found at the North Carolina State Extension.
Citation: Judelson H. 2007. Sexual Reproduction in Plant Pathogenic Oomycetes: Biology and Impact on Disease, p 445-458. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch27