It is also suggested that to prevent the spreadof Phytophthora spp. from nursery stock to previously uninfected orchards, roots of potted trees should be inspected and tested for the presence of Phytophthora spp.. Even after planting healthy nursery stock, the application of fungicides is necessary in part because young, fibrous roots are more susceptible to root rot than older roots and it is suggested that fungicides, together with nematicides or a soil fumigant be used to protect replants during the first two years of growth, especially if the orchard has a history of Phytophthora problems. The buildup of inoculum during root rot outbreaks can quickly grow and during rain or irrigation events. Sporangia or zoospores, sometimes together, may be splashed up to low-hanging fruit and cause outbreaks of brown rot. Brown rot develops as light brown, leathery lesions that expand until the whole fruit turns brown. Diseased fruit have a characteristic pungent odor and at high humidity, the fruit surface is eventually covered by distinct white mycelium. Both the root rot and brown rot phases are damaging, though while fruit brown rot leads to immediate crop loss,maceta 25 litros root rot outbreaks instead results in a slow decline of infected trees.
Trees infected by Phytophthora spp. eventually show reduced vigor and production. Systematic die back of the canopy occurs when uptake of water and nutrients is restricted due to poor root health caused by the decay of feeder roots. Following infection of the roots, the root cortex is eventually degraded, leaving only the inner stele and giving the root system a stringy appearance. This underlines the importance of root rot management in both an integrated brown rot management program to prevent both the build-up of inoculum in citrus orchards and as a means to maintain a productive orchard. Currently the control of root rot and brown rot is addressed through integration of cultural practices and fungicide applications. Among cultural practices, the usage of resistant root stocks has long been utilized for the management of Phytophthora root rot and gummosis issues. Trifoliate and hybrid rootstocks such as C-35 and C-32 citrange are tolerant of root rot and gummosis. Other citrange rootstocks such as Carrizo and Troyer citrange are intermediate in their tolerance to root rot but are tolerant of gummosis. Irrigation management is another important facet to controlling the spread of Phytophthora root rot, brown rot and gummosis. It has previously been shown that irrigation management that avoids over-watering can significantly reduce populations of Phytophthora species in the soil by preventing saturated soil conditions that are conducive to sporangia formation, zoospore release, and zoospore motility. To promote consistent drainage and reduce the presence of saturated soil, trees can be planted on raised berms and micro-sprinkler irrigation systems installed to replace furrow irrigation which easily over saturates the soil.
Additional in-season and preharvest fungicide applications, however, are often still needed to control root rot, especially in the establishment of new orchards, and to manage brown rot during the rainy winter harvest season. Preharvest applications of copper or phosphonates are used to manage brown rot, and phosphonate or phenylamide fungicides are applied to the soil to reduce root rot. The limited number of chemistries available for controlling Phytophthora diseases and few options for fungicide rotations has led to their overuse and resulted in the development of resistance. Several formulations of the phenylamide fungicides metalaxyl and mefenoxam have been used by growers for many years to control root rot and gummosis since their introduction. It is known that Phytophthora species can develop resistance to this fungicide class after repeated exposure. It has also been observed that phenylamide-resistant populations tend to decrease in the absence of selection pressure. This suggests that reducing usage of phenylamide-containing products could eventually make this fungicide class effective again when used strategically in a resistance management program. The new chemistries ethaboxam, fluopicolide, mandipropamid and oxathiapiprolin all possess unique modes of action, all different from one another and from currently registered products. The availability of new modes of action will facilitate the implementation of resistance management strategies using fungicide rotations that will minimize the risk of resistance development.
As a result of our previous work , oxathiapiprolin has since been federally and state registered for usage on citrus, while the remaining fungicides are still pending registrations for field application use. Once these fungicides are registered, the implementation of new fungicide rotations with existing chemistries will help not only to manage Phytophthora diseases but also slow the development of resistance, maximize the lifespan of available chemical treatments, and help ensure continued trade with foreign markets by reducing the prevalence of Phytophthora root rot and brown rot in orchards, effectively lowering the disease incidence to below detection levels. Thus, the objectives of the following studies were to: Identify the efficacy of new fungicides ethaboxam, fluopicolide, mandipropamid and oxathiapiprolin, at inhibiting several different life stages of Phytophthora spp. known to cause root rot, and to determine baseline sensitivity references for future resistance monitoring efforts using isolates of several Phytophthora spp. collected from throughout California citrus production regions; evaluate these new compounds in both greenhouse and field settings to verify their efficacy as potential root rot treatments and determine potential growth promotion benefits of the fungicides and ascertain a better understanding of the properties of oxathiapiprolin in citrus plant systems following application to determine if oxathiapiprolin is capable of systemic translocation for the purposes of refining potential application practices and improve management of Phytophthora diseases common to citrus within California. Phytophthora spp. are plant pathogens in the Kingdom Stramenopila that cause diseases on a wide variety of cultivated and non-cultivated plants. Numerous species have been associated with citrus crops worldwide ,maceta 10 litros and in California, P. citrophthoraLeonian, P. syringaeKleb., P. nicotianae Breda de Haan , and P. hibernalis Carne are present. These species can cause fruit brown rot , root rot , foot rot , and trunk cankers or gummosis.
Overall, P. hibernalis is the least commonly encountered species in California. P. nicotianae is most prevalent from late spring to early fall, P. citrophthora can be found year-round, whereas P. syringae and P. hibernalis are restricted to the cooler seasons. Recently, P. syringae and P. hibernalis have been designated quarantine pathogens in China after they were detected in citrus fruit shipments from California. This has restricted the California citrus trade with this important export market, causing high economic losses to the state’s citrus industry. The disease phases of Phytophthora species on citrus are closely interrelated. For example, a high incidence of root rot will lead to the buildup of zoospore inoculum in thesoil that may be splashed by rain or irrigation onto low hanging fruit, causing brown rot. Vice versa, infected fruit that fall to the ground will increase the amount of inoculum in the soil. Therefore, management of Phytophthora diseases should be done in an integrated approach, including cultural practices , and preand post harvest fungicide applications. Few fungicides are currently registered in the United States for the control of Phytophthora diseases of citrus. Copper products and phosphonates are used in fruit and foliar applications to manage brown rot and gummosis, whereas phosphonate and phenylamide compounds are applied to the soil to reduce root rot. Potassium phosphite was registered in 2013 as the first post harvest fungicide to manage brown rot. The limited number of fungicides available has led to their over-use, and in subsequent resistance development. Resistance to the phenylamide class of fungicides has been reported in Oomycota pathogens of numerous crops , including citrus where resistant populations of P. nicotianae are established in Florida nurseries. Resistance was recently also reported for potassium phosphite in isolates of P. citrophthora and P. syringae collected from California citrus orchards, and brown rot caused by these isolates could not be controlled using registered rates of potassium phosphite. New fungicides are becoming available to manage Phytophthora diseases of citrus with the pending registrations of four compounds with modes of action different from those of registered fungicides : the thiazole carboxamide ethaboxam, the benzamide fluopicolide, the carboxylic acid amide mandipropamid, and the piperidinyl thiazole isoxazoline oxathiapiprolin. These new chemistries each have unique and different modes of action. Mandipropamid is suspected to target a cellulose synthase gene vital to cell wall biosynthesis. Ethaboxam inhibits mitosis and cell division by disrupting microtubules. Fluopicolide delocalizes spectrin-like proteins, thereby destabilizing plasma membrane formation. Oxathiapiprolin inhibits oxysterol-binding proteins affecting all stages in the asexual life cycle.
The introduction of four new fungicides with unique modes of action provides an opportunity for designing new effective disease control programs and to implement resistance management strategies that slow the development and spread of resistance while extending the lifespan of existing treatments. To provide a reference for future resistance monitoring and to possibly use the new fungicides against the most sensitive life stages of Phytophthora spp., we determined the in vitro toxicities of these compounds. Thus, the objectives of this study were to evaluate in vitro sensitivities of mycelial growth of P. citrophthora, P. syringae, P. nicotianae, and P. hibernalis to ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin and compare to mefenoxam, and assess the toxicity of these fungicides to other selected stages in the life cycles of Phytophthora species on citrus in California. The ubiquitous occurrence of Phytophthora species in citrus growing areas worldwide often requires integrated management strategies to successfully establish and maintain orchards in order to produce a high-quality crop. In California, a high level of disease control is also required to minimize detection of any fruit with brown rot upon arrival in export markets where P. syringae and P. hibernalis are quarantine pathogens. Quarantine laws prohibit the movement of diseased commodities containing pathogens not present in the import country. The use of cultural practices such as irrigation methods that minimize wetting of fruit, removal of lower branches, and harvesting above a selected height to exclude fruit exposed to splashing water and soil contamination provide some level of brown rot management. Still, the use of fungicides has become an integral part of citrus production in preventing fruit from developing brown rot during transit to the port of arrival of the export market. This, however, is hampered by the current limited number of treatments available and the development of resistance to the phenylamides and more recently to the phosphonates in Phytophthora spp. populations. The pending registration of the new active ingredients ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin will be an important step to increase treatment options for improved disease management and to reduce the risk of resistance development due to overuse of any single mode of action. The Fungicide Resistance Action Committee considers the resistance risk of ethaboxam and mandipropamid as low to medium and of oxathiapiprolin as medium to high, whereas the risk for fluopicolide is currently not known. This is the first study comparing in vitro toxicities of these new Oomycota fungicides to important stages in the disease cycles of Phytophthora species from major citrus production areas in California. Mycelial growth responses were evaluated for all isolates included in the study because mycelium is the main somatic structure that is capable of surviving from season to season in infected plant material if climatic conditions allow. Sporangia are readily formed by the four species, and effects of the fungicides on their production were evaluated using P. citrophthora as a representative. Sporangia produce zoospores that are the main infective propagules of Phytophthora species, and abundant production is a major cause of disease epidemics during rain and irrigation events when zoospores are disseminated from the soil or from infected fruit onto low hanging fruit. Therefore, we also evaluated rate effects of the fungicides to determine the response of sporangia to lower concentrations.The effect of the fungicides on their production was investigated using P. nicotianae where these structures are known to occur and are considered critical for persistence during the colder winter months. Oospores are also produced by the homothallic P. syringae and P. hibernalis. Previously, large quantities of oospores in fallen apple leaf litter were found in outbreaks of P. syringae in apple orchards in the United Kingdom, suggesting the importance of oospores in pathogen survival.