Podosphaera macularis facts for kids
Quick facts for kids Podosphaera macularis |
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| Scientific classification | |
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Podosphaera
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| Species: |
macularis
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| Synonyms | |
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Alphitomorpha macularis |
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Podosphaera macularis (formerly Sphaerotheca macularis) is a plant pathogen infecting several hosts including chamomile, caneberrie, strawberries, hop, hemp and Cineraria. It causes powdery mildew of hops.
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Host range and symptoms of Podosphaera macularis
The pathogen that causes powdery mildew of hops was once considered to be Sphaerotheca macularis, which is capable of infecting many plants; however, in recent years, the pathogen that causes powdery mildew of hops has been taxonomically classified as Podosphaera macularis. This ascomycete is only known to be pathogenic on hop plants, including both ornamental and wild hops. The host range of many Podosphaera macularis strains is restricted by the existence of resistant hop varieties, such as the “Nugget” variety of Washington state and Oregon, although in recent years, resistance within this hop variety has been overcome in the laboratory. When disease does occur, early symptoms include chlorotic spots on the leaves of hop plants. Spots may fade to gray or white as the season progresses. Signs include white clusters of hyphae, which are often present on the leaves, and in some cases can infect the cone itself. If this infection occurs, a brown, necrotic lesion may develop. When both mating types exist within a population, chleistothecia can form and are visible as small, black dots on the undersides of leaves.
Optimal environment
Under optimal conditions, this polycyclic disease can potentially grow 20 generations in a growing season. Favorable environmental conditions for Podosphaera macularis fecundity include low sun exposure, soil moisture, and excessive fertilization. The optimal temperature range for spore and mycelium growth is 18 to 25 °C. In addition, periods with small temperature differences between night and day, with a minimum of 10 °C at night and a daily high of 20 °C increase the risk of infection. High humidity and optimal temperature conditions are necessary for primary infection between the middle and end of May. The cleistothecia swell up and burst due to increased turgor pressure leading to the release of ascospores. During the secondary infection period from mid-July to August, conidia infectivity and germination is highest around 18 °C. However, leaf wetness is not essential for the formation and germination of conidia, but rather slight rain has an indirect effect related to high humidity and low sun light. Since the life cycle mainly exists externally, with only haustoria inside the host, supra-optimal temperatures and low relative humidity are unfavorable parameters for germination, infection or sporulation of powdery mildew. Temperatures exceeding 30 °C for more than three hours reduce the chance of infection by up to 50%. Intense rain and wind periods that cause spores blown throughout the hop yard also prevent powdery mildew fecundity. In addition, solar irradiation can kill released spores, but as hops grow, the sun can't penetrate the dense canopy.
Disease importance
In 1997, hop powdery mildew was reported for the first time in hop yards in the United States Pacific Northwest. In Washington, severe infections lead to a yield loss of 800 hectares (US $10 million) of crops. At the time, sulfur was the only registered pesticide used on hop that was effective against powdery mildew. In 1998, the disease was confirmed in Idaho and Oregon. As a result, Yakima Valley growers managed the disease using approaches developed in Europe, such as, labor-intensive cultural practices, mechanical or chemical removal of spring growth, and intensive fungicide programs despite the small number of fungicides available for hop at the time. Although the methods successfully limited disease development, the depressed market for hops couldn't sustain the expensive production costs ($1400/ha annually in 1998). In 2001, a contracting brewery rejected 50% of an aroma hop grown in Oregon because of cone browning after drying, resulting in an additional US $5 million in losses that year. These losses have contributed to economic depression in the hop market and have forced several growers to declare bankruptcy. Currently, hop powdery mildew exists annually in all production regions in the United States. While more research is necessary to understand Podosphaera macularis and control, the current management system has returned economics to hop industry. Disease levels have decreased and control costs have been reduced to $740/ha on average. Unlike New York and California, hop production in the Pacific Northwest is likely to continue.
See also
In Spanish: Podosphaera macularis para niños
