Plant Diseases
ABOUT THE COLLECTION
Arizona Cooperative Extension is an outreach arm of The University of Arizona and the College of Agriculture and Life Sciences (CALS). The repository collection includes current and historical Extension publications on these topics: Animal Systems; Consumer Education; Farm Management and Safety; Food Safety, Nutrition and Health; Gardening/Home Horticulture; Insects and Pest Management; Marketing and Retailing; Natural Resources and Environment; Plant Diseases; Plant Production/Crops; Water; and Youth and Family. Current publications are also available from the Cooperative Extension Publications website.
QUESTIONS?
Contact College of Agriculture and Life Sciences Publications at pubs@cals.arizona.edu.
Recent Submissions
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Información de germinación para especies comunes de restauración de ArizonaLa restauración ecológica basada en semillas es un enfoque utilizado para revegetar hábitats dañados y perturbados mediante la dispersión de semillas con la expectativa de que ocurra la germinación y las plantas se establezcan y prosperen. Aunque la restauración puede mejorar la salud y productividad de los paisajes al revitalizar los servicios ecosistémicos tanto directa como indirectamente, lograr una restauración exitosa es difícil de lograr, especialmente en sistemas áridos (Copeland et al., 2018). La germinación es un cuello de botella bien conocido para el crecimiento de las plantas que dificulta una restauración exitosa (James et al., 2011).
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Vegetable Diseases Caused by Phytophthora capsici in ArizonaPhytophthora capsici is responsible for some of the most important diseases of peppers and cucurbits worldwide. The diseases are commonly referred to as Phytophthora blight, Phytophthora crown and root rot, Phytophthora fruit rot, and damping off. P. capsici was first described by Leon H. Leonian on chile pepper in New Mexico in 1922. This pathogen subsequently has caused severe disease epidemics on susceptible vegetables in North, Central and South America, Europe, Asia, and many states across the United States. In Arizona, the disease has become a chronic problem for pepper, squash, and melon production in the southern part of the state. P. capsici can cause extensive losses on peppers and cucurbits under favorable weather conditions during the summer monsoon season.
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Pecan Bacterial Leaf ScorchPecan bacterial leaf scorch (PBLS) is an important and chronic disease that affects pecan in Arizona, as well as other pecan production regions of the United States. This disease was first noticed throughout the southeastern United States in 1972 and mistakenly thought to be a fungal disease. In 2000, the disease was correctly identified as a bacterial disease known as PBLS, impacting mostly the eastern growing region of the US. Currently, there are more than 25,000 acres, of which 14,000 is in fruit bearing age. Pecan acreage continues to grow by 2,000 acres annually. In 2015, the AZ pecan industry contributed more than $54 million dollars to the state’s economy. Pecan bacterial leaf scorch was first detected in 2015 not only in Arizona, but also in California, New Mexico, and Texas. PBLS is a primary concern for pecan trees under stressful conditions including nutritional, physiological, and environmental stress. PBLS is a rapidly emerging disease to Arizona pecan industry and this document is designed to help pecan growers answer commonly asked questions about PBLS.
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Phymatotrichopsis Root Rot in PecanPhymatotrichopsis root rot (abbreviated as PRR) is commonly referred to as Cotton root rot, Phymatotrichum root rot, Texas root rot, or Ozonium root rot. This disease is caused by a soilborne fungus Phymatotrichopsis omnivora (abbreviated as PO) that attacks the roots of susceptible plants, causing sudden wilt and death. This pathogen has a wide host range and can attack more than 2,300 dicotyledonous (broadleaf) plant species including many ornamental and important agronomic crops (Lyda, 1978). In contrast, the fungus colonizes but does not kill monocotyledonous plants (grasses), which are highly tolerant or resistant to this pathogen. Phymatotrichopsis can be found in a wide range of soils, but is more prevalent in calcareous clay soils with a high pH range of 7.0 – 8.5. It is one of the most destructive fungal pathogens of pecan, pistachio, cotton, alfalfa, grape, fruit trees, shade trees, and ornamental plants like conifers. The economic losses attributable to this important disease are over hundreds of millions of dollars annually in the southwestern and south central United States (www.cotton.org).
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Cotton Stem Blight and Boll RotCotton stem blight and boll rot is caused by the necrotrophic fungus Sclerotinia sclerotiorum (Hu et al. 2018). This pathogen can attack hundreds of plant species including many important agricultural crops such as vegetables, legumes, sunflowers, canola, many flowering bedding plants, and stone fruits (Kohn 1979). The disease caused by Sclerotinia spp. is commonly referred to as white mold. The disease is favored by cool temperature (between 59 and 75 degrees Fahrenheit) and moist conditions under a closed plant canopy. S. sclerotiorum is a soil-borne fungus that usually infects the lower stem and other foliage tissues near the soil surface and can attack roots of some plant species. However, the primary impact of white mold on cotton is manifested in yield losses, although cotton stand establishment and seed quality could be affected by this disease. Due to wet and cool weather conditions during the “monsoon” season in the southeast Arizona, fields with a history of bean production will have an increased risk of cotton boll rot and stem blight.
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Stripe Rust of Small GrainsStripe rust was reported for the first time on barley in Arizona in 1993. Stripe rust could develop into a serious disease in Arizona if it becomes established on native grasses and weather conditions allow the fungus to proliferate. The disease is caused by the basidiomycete fungus Puccinia striiformis. Stripe rust appears as yellow orange stripes between the veins of the leaf blades. The disease increases water use, shrivels the grain, and decreases yield. The fungus is an obligate parasite because it can survive and reproduce only on live hosts. Mild winters and cooler wet weather in the spring favor development of the disease. Host plant resistance is the most effective means to manage the disease. However, the fungus evolves rapidly and different races can occur from one year to the next and might overcome resistance. A number of fungicides are labeled to control the disease but scouting and early detection are crucial for timely applications of fungicides to achieve effective control.
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Verticillium Wilt of CottonVerticillium wilt is one of the most important diseases of cotton worldwide. This disease was first reported on Upland cotton in Virginia in 1918. Currently, Verticillium wilt is widespread in most of the cotton belt region of the U.S. and causes great economic losses in some cotton producing areas such as western Texas and San Joaquin valley of California. In Arizona, Verticillium wilt occurs only on Upland cotton, especially more severe in the regions at elevations above 3,500 feet in Cochise, Graham, and Greenlee counties where cooler air temperatures (70 to 80 °F) are more favorable for symptom expression from late summer to fall. A recent study showed that overwatering and heavy soil with higher composition of silt and clay led to an increased in disease incidence (Land et al., 2017). With adoption of tolerant cultivars, Verticillium wilt has not been a major problem in Arizona cotton in the recent past.
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Verticillium Wilt of PistachioVerticillium wilt is a common vascular wilt disease of many economically important crops. The disease was first identified in pistachio in California in 1970s and historically has been a serious problem of pistachio. With the introduction of resistant rootstocks, the significance of Verticillium wilt has diminished greatly in the US. In Arizona, Verticillium wilt can be a significant problem of older pistachio trees on susceptible rootstocks (Pistacia atlantica and P. terebinthus) in Cochise county. The disease often is lethal to trees and tree death causes significant yield loss and incurs high replanting costs.
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Cotton Root RotCotton root rot is a serious soilborne fungal disease of cotton and numerous dicot plants in the south-western United States and northern Mexico. The disease is also known as Texas root rot, Phymatotrichopsis or Phymatotrichum root rot, and Ozonium root rot.
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Pythium Crown and Root Rot of Industrial HempThe 2018 U.S. Farm bill changed the Controlled Substances Act and legalized cultivation of industrial hemp in the United States. In Arizona, the AZDA issues licenses and regulates the production and processing of industrial hemp. In 2019, 165 growing licenses and 43 nursery licenses were issued across the state, resulting in the planting of 5,430 acres in 13 counties. The University of Arizona, Extension Plant Pathology Laboratory, in Tucson has detected several major diseases in hemp crops grown during the 2019 – 2020 seasons. In this publication we will describe one such disease, Pythium crown and root rot. This disease has been observed in hemp grown with plastic mulch film and saturated heavy soil, and is associated with over-irrigation, poor drainage, and high temperatures. Pythium crown and root rot does not appear to be isolated to Arizona, and also has been reported in field-grown hemp in North Carolina, Indiana, California, and Nevada.
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Phytophthora Rots of Apple and PistachioFruit and nut trees are economically important crops in Arizona backyards or commercial orchards. The most common and most important diseases observed in fruit and nut trees are root diseases that cause a gradual and irreversible decline of the tree. Phytophthora rots of root, crown (rootstock) and collar (scion) are common and destructive diseases of fruit and nut trees worldwide. In Arizona, apple, pistachio, peach, and citrus are susceptible to Phytophthora rots, especially trees grown in heavy soils or under wet soils conditions resulting from excessive irrigation for extended periods.
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Pierce’s Disease of GrapeVinifera grapes have become economically important crop in Arizona, as wine tourism contributed $56.2 million to the state economy in 2017. The acreage of vinifera grapes in Arizona grew from 950 to more than 1,300 acres between 2013 and 2016. In 2019, Arizona grape acreage has grown to approximately 2,000. Pierce’s disease (PD) is endemic to the Southern U.S. and is a threat to vine production in Arizona, especially in regions where winters are warm.
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Phymatotrichopsis Root Rot of GrapePhymatotrichopsis root rot of grape (PRR) is commonly known as cotton root rot, Texas root rot or Phymatotrichum root rot. PRR is a destructive disease of many economic crops including wine grape grown in calcareous clay soils with a high pH range of 7.0 – 8.5 in central and southern Arizona. Significant number of vine losses to PRR occur annually in many vineyards in Cochise, Santa Cruz, and Yavapai counties.
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Alfalfa Mosaic Virus (AMV) Infections in Garbanzo BeansAMV was first identified on lucerne in the USA and now poses a significant threat to worldwide production of garbanzo beans. AMV has infected garbanzo beans in Arizona and California. In April of 2018, a number of garbanzo fields in central Arizona were heavily infected by AMV and resulted in near complete crop failure. Depending on virus-cultivar combination and the stage of growth at infection, severe symptoms can be caused by infection with AMV and yield loss can vary from complete crop failure to decreases in grain yield and quality.
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Symptom Identification and Management of Cotton Seedling DiseasesSeedling diseases, also known as damping-off (seedling death), are caused by several common soil-inhabiting fungi acting alone or collectively during pre-emergence and postemergence of cotton seedlings. Pre-emergent damping-off refers to rot and death of germinating seeds prior to emergence from the soil. Post-emergent damping-off refers to seedling death after emergence from the soil. Seedling diseases are common but often a minor problem in cotton production areas of Arizona in most years. However, significant stand loss to seedling diseases can occur sporadically in some fields without good crop rotation history, especially when cool, wet weather conditions exist during the first weeks after planting that allow soil temperatures to drop below 65°F. Consequently, growers may have to replant parts or whole fields incurring substantial costs including seed, fuel, labor, additional costs of late season pest control as well as experience yield reductions due to late planting.
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Alternaria Leaf Spot of CottonAlternaria leaf spot of cotton is also known as Alternaria leaf blight. The disease was first identified in cotton in the US in 1918 and is now distributed worldwide. Alternaria leaf spot has been considered a minor disease in the cotton growing areas of Arizona. The disease is frequently associated with senescing tissue of cotton under physiological stress (heavy boll load) or nutritional stress (potassium deficiency) late in the growing season. On rare occasion it can also affect seedlings. In recent years, several disease outbreaks that led to severe defoliation in late-season cotton were reported from Graham County. The disease can severely affect susceptible Pima cotton varieties and also attack upland cotton varieties. Susceptible varieties have had nearly 100% of leaves infected in years when weather conditions are conducive for disease infection and development. The impact on yield in Arizona was estimated to be 10% to 15% in highly susceptible cotton varieties.
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Fusarium Wilt of CottonFusarium wilt is a destructive vascular wilt and root rot of many plant species, including all species of domesticated cotton. The disease was first described in Alabama cotton fields in 1892 and is now widespread in most states across the US Cotton Belt and throughout the world. Fusarium wilt often kills susceptible plants in a heavily infested field. It may cause significant yield losses to growers.