Podosphaera macularis

Host range and symptoms of Podosphaera macularis 

The pathogen that causes sphaerotheca macularis , which is capable of infecting many plants; However, in recent years, the pathogen has been shown to be highly significant in the past, and has been taxonomically classified as Podosphaera macularis . ^[1] This ascomycete is only 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 varieties hop, 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. ^[2]When does chlorotic spots occur 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. Occasionally, chleistothecia are visible as small, black dots on the undersides of leaves.

Disease cycle

Podosphaera macularis overwinters on the soil surface in debris as fungal survival structures ( chasmothecia ) or as mycelia in plant buds. ^[3] These chasmothecia are formed closer to the end of the growing season. The characteristic morphology of chasmothecia of Hop Powdery Mildew are spherical black structures with spiked appendages. When the conditions are encountered during early spring, the asci (sac-like structures) within the chasmothecia will rupture and ascospores will be discharged. Specifically, the favorable conditions for ascospore release include low light, high fertility, and high soil moisture. ^[3]Additionally, optimal infection is observed when the temperature is between 18 and 25 ° C. ^[4] Furthermore, the ascospores act as the primary inoculum and are dispersed passively by wind. Upon encountering a susceptible host plant, the ascospores will germinate and cause infection. Following infection, masses of asexual spores ( conidia ) will be produced during the season. ^[3] It is these masses of conidia that contribute to the characteristic white, powdery appearance of infected plants. The lower leaves are the most affected, but the disease can be seen on the ground. ^[4] These conidia are dispersed through wind. Thus, Podosphaera macularisare a polycyclic pathogenase conidia are produced / dispersed during the growing season. Particularly, the disease will be noticeable on infected plants as soon as possible at 12 to 15 ° C compared to 5 days at 18-27 ° C. ^[5]These spore-covered shoots that emerge from infected buds are called “flag shoots” ^[4] and will be stunted with distorted leaves. Periods of rapid growth are the most favorable for infection. In addition, the period in which lateral development is taking place within the plants is also very vulnerable to the development of the disease. ^[4] Due to Podosphaera macularis local disease infection, only the location of the host tissue where spores have been developed.

Optimal Environment

Under optimal conditions, this polycyclic disease can potentially grow in a growing season. ^[6] Favorable environmental conditions for Podosphaera macularis fecundity include low sun exposure, soil moisture, and excessive fertilization. ^[7] The optimum temperature range for spore and mycelium growth is 18 to 25 ° C. ^[6] In addition, with a minimum temperature of 10 ° C and a daily high of 20 ° C increase the risk of infection. ^[8]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. ^[6] However, leaf wetness is not essential for the formation and germination of conidia, but it has an indirect effect on high humidity and low sunlight. ^[8]Since the life cycle is predominantly occurring externally, with only one haustoria inside the host, there is no evidence of favorable conditions for germination, infection or sporulation of powdery mildew. Temperatures exceeding 30 ° C for 50% chance of infection. ^[7] Intense rain and wind periods that cause spores blown throughout the yard also prevent powdery mildew fecundity. In addition, solar irradiation can kill released spores, but as hops grow, the sun can not penetrate the dense canopy. ^[8]

Management

The two primary ways to control Podosphaera macularis are cultural and chemical control. The most effective way to manage a powdery mildew is through preventative measures. Cultural control of the disease include growing powdery-mildew tolerant / resistant varieties of the host plant. ^[4] Cultural practices that can help prevent the disease include careful monitoring of water and nutrients, reducing initial inoculum, and removing basal growth. Furthermore, pruning, crowning, and / or scratching will help in the further development of the disease. ^[4]Pruning consists of removing shoots before training. Crowning refers to the process of removing the top 1-2 inches of the crown before budbreak. Scratching is done through disturbing the soil 1-2 inches of buds. ^[4] All of these methods disturb the overwintering stage of the life cycle of Podosphaera macularis . Likewise, chemical control primarily consists of spraying fungicides in hopes of preventing the disease through the use of early, continuous spraying during the growing season. Thus, prophylactic fungicide programs can be very effective in preventing disease. ^[3]Since the fungicides are a preventative measure, they are not very useful during a full-blown infection. Therefore, the use of fungicides disturbs release of spores and further infection in the disease cycle of Podosphaera macularis . As there are several fungicides that are effective against powdery mildew, it is important to apply the fungicides at specific times. If it is known that powdery mildew is present, it should be started soon. Due to the fact that they are rapidly increasing their resistance to fungicides, it is important to rotate the fungicides that are used. ^[9] However, these applications should be used during such development. ^[3] In this case, removing basal growth before flowering and applying a protective fungicide with long-term residual action should be employed.

Disease Importance

In 1997, hop powdery mildew was reported for the first time in the United States Pacific Northwest. In Washington, severe infections lead to a yield loss of 800 hectares (US $ 10 million) of crops. ^[10] At the time, the pesticide was used only when it 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 removal of spring growth, and intensive fungicide programs for the time. ^[10]Although the methods were successfully limited, the depressed market for hops ($ 1400 / ha annually in 1998). ^[10] In 2001, 50% of an aroma was grown in Oregon because of cone browning after drying, resulting in an additional US $ 5 million in losses that year. ^[11] These losses have been contributed to the growth of the market by the market. ^[11] Currently, hop powdery mildew is annually in all production regions in the United States. While more research is needed to understand Podosphaera macualarisand control, the current management system has returned to the industry. Disease levels have decreased and control costs have been reduced to $ 740 / ha on average. ^[11] Unlike New York and California, production in the Pacific Northwest is likely to continue.

Pathogenesis of Podosphaera macularis 

In order for pathogenesis to occur, a viable pathogen, susceptible host, and conducive environment must simultaneously be present. The germ tube of P. macularisplays an important role in determining the pathogen’s viability, because it can penetrate its host in approximately 15 hours. The germ tube begins branching, leading to as many as three potentially conidia-forming germ tubes. As the pathogen invades host tissue, it establishes a haustorium to facilitate the collection of nutrients from the host cells. Despite this, only certain hosts are susceptible, because there are seven R genes in hop varieties that can be activated in response to infection. Many of them operate with the initial haustorium to lyse, or by preventing the pathogen from spreading. The spread is halted by a hypersensitive response, which is often associated with the establishment of large callose and lignin deposits surrounding infected cells. Although susceptible to increase in response to infection, the hypersensitive response is only one. Finally, powdery mildew can grow in a relatively hot and dry environment compared to downy mildew, conidia production peaks at temperatures of approximately 20^o C. Conidia Can Be Produced at temperature Above 25 ^o C, infectivity Their goal is Often reduced. ^[6]

External links

• Fungorum Index
• USDA ARS Fungal Database

References