Issue 6, July 22, 2025

Taylor Klass, Lead Diagnostician

Jason Pavel, Diagnostician

Cucurbit downy mildew is a plant disease that can spread rapidly under ideal conditions. It is caused by the oomycete/water mold Pseudoperonospora cubensis and impacts cucumbers, melons, pumpkins, and squash. Note that there are two unique groupings of P. cubensis isolates, clade 1 and clade 2. Clade 1 prefers watermelon, pumpkin, and squash, and clade 2 prefers cucumber and cantaloupe. Therefore, it is possible to have a cucumber plant infected with downy mildew right next to a healthy watermelon plant that is not impacted by the disease. The pathogen does not overwinter in Arkansas, so each year it needs to re-establish through airborne spores coming from further south in regions where cucurbits are grown year-round. The spores can survive up to 16 days in the air and can travel hundreds of miles.

Symptoms usually begin on older leaves 4 – 12 days after infection and appear as angular yellow spots that are bound by leaf veins. These spots often become brown and necrotic as the disease progresses. On the undersides of the leaves, especially under moist conditions, you can often see a fuzzy grayish or purplish sporulation — this is the pathogen producing new spores. While the spots on watermelon leaves may not always have the angular appearance one might expect, the leaves will often have an exaggerated upward curl to them and the fuzzy sporulation on the underside should look the same as other cucurbits. The disease typically starts in the lower canopy and spreads upward, leading to defoliation, poor fruit development, and yield loss if not controlled promptly.

Downy mildew thrives in cool, humid conditions. While the pathogen can infect plants at temperatures between ~40 – 85°F, ideal temperatures are around 60 – 75°F with extended leaf wetness from rain, dew, or overhead irrigation. If we are going to have issues with this disease in Arkansas, it commonly appears in early to mid August, especially during cloudy, wet spells. While we haven’t seen the disease yet this year, consider this a reminder to stay vigilant and keep it in mind as we move into August and September. Because of cucurbit downy mildew’s explosive nature and resistance to many fungicides, early detection and immediate action are critical. We cannot overemphasize the importance of scouting your fields and plants!

Management of cucurbit downy mildew relies heavily on planting varieties that are more tolerant to the disease, timely fungicide applications, and cultural practices. Cultural practices, such as improving airflow and reducing leaf wetness, can reduce disease pressure. Climbing cucurbits can be trellised to increase airflow, while heavier crops can be spaced further apart on wider rows. Once the disease is established, it is very difficult to control, making prevention and weekly scouting essential.

Keep any eye on the Cucurbit Downy Mildew Forecasting website (cdm.ipmpipe.org) and sign up to receive alerts when cucurbit downy mildew cases have been confirmed near you. For commercial growers, there are several fungicide options for control of cucurbit downy mildew, including Elumin (ethaboxam, FRAC 22), Orondis Ultra (oxathiapoprolin FRAC 49 + mandipropamid FRAC 21), Orondis Opti (oxathiapoprolin FRAC 49 + chlorothalonil FRAC M05), and Ranman (cyazofamid FRAC 21). For the complete list of options, please see page 144 of the 2025 MP154 Arkansas Plant Disease Control Products Guide. Read and follow the directions on any fungicide product label. Clade 2 is especially quick at developing resistance, so make extra sure to rotate fungicides with different FRAC codes when treating for downy mildew on cucumbers or cantaloupes.

Home gardeners do not have fungicide options available for specific control of cucurbit downy mildew. However, fungicides containing chlorothalonil or copper may help provide some control (see pages 156 – 157 of the 2025 Arkansas Plant Disease Control Products Guide). Home gardeners still have a significant role to play in cucurbit downy mildew management for the state of Arkansas, as the pathogen could spread via wind from a home garden to a commercial field. If you think your cucurbit crop is infected with downy mildew, contact your local county extension agent! For further reading on previous cucurbit downy mildew occurrences in Arkansas, please see the 2023 and 2020 updates from Dr. Aaron Cato, UADA Extension Horticulture IPM Specialist.

Downy Mildew Symptoms on a Cucumber Leaf

Photo by Nancy Gregory, University of Delaware, Bugwood.org

More Advanced Downy Mildew Symptoms on a Cucumber Leaf

Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Signs (Gray Sporulation) of the Downy Mildew Pathogen on the Underside of a Cucurbit Leaf

Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Downy Mildew Symptoms on a Watermelon Leaf

Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Downy Mildew Symptoms and Signs on a Cucurbit Leaf

Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org



Conidia (asexual spores) and Conidiophore of Pseudoperonospora cubensis, the Causal Agent of Downy Mildew on Cucurbits

Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Spider mites are tiny arthropod pests (in multiple genera in the family Tetranychidae) that can cause significant damage to a wide variety of plants, especially during hot, dry conditions. These mites feed on leaves by piercing plant cells and sucking out the contents, which causes tiny yellow or white stipples on the upper leaf surface. As feeding continues, leaves can become bronzed, dry, and eventually drop off. Heavy infestations may also include fine webbing that covers leaves and stems — a classic sign of spider mite activity.

Damage from spider mites can reduce photosynthesis, weaken the plant, and make it more vulnerable to other stresses. In vegetables like tomatoes, beans, or cucurbits, severe infestations can lead to reduced yields and poor fruit quality. In ornamentals, mites can cause leaf drop and aesthetic damage. Because mites reproduce rapidly — especially in warm weather — populations can explode if not monitored and managed early.

Management of spider mites starts with regular scouting, especially on the undersides of leaves. A simple way to check is to tap leaves over a white sheet of paper and look for tiny moving dots. Alternatively, use a hand lens or magnifying glass to look on the underside of leaves (mites cannot easily be seen with the naked eye). If mites are found, the first step is to reduce plant stress: keep plants well-watered (but not over-watered, as that will lead to other issues), remove infested leaves, and spray down foliage with water to knock mites off and increase humidity (which they dislike). Keep in mind that prolonged leaf wetness can favor additional plant problems like fungal or bacterial infections, so spray the foliage at a time of day when the leaves should dry quickly. Spraying during a time of day when the leaves aren't shaded and the sun won't soon be setting, such as in the morning, is ideal.



For more severe infestations, horticultural oils, insecticidal soaps, or miticides labeled for spider mites can be effective, though repeat applications are typically needed. Most miticides do not affect mite eggs. Be sure to thoroughly coat both sides of the leaves. Homeowners can use insecticidal soap or malathion products such as Ortho MAX Malathion Insect Spray or Bonide Malathion Insect Control. Commercial growers have more miticide options available. See the 2025 MP 144 Insecticide Recommendations for Arkansas, focusing on the section for your specific crop. As always, follow all directions on any product label.  Spider mites frequently develop resistance to various miticides, so rotating miticides can be important for proper control. Avoid broad-spectrum insecticides, as they can kill natural predators like predatory mites and lady beetles, which help keep mite populations in check. Encouraging these natural enemies and rotating miticide products with different modes of action can prevent resistance and keep spider mites under control.

Webbing Produced by the

Spruce Spider Mite

Photo by Sherrie Smith, University of Arkansas Cooperative Extension Service

Adult Spider Mites on a Plant Leaf

Photo by David Cappaert, Bugwood.org

Spider Mite Damage (Stippling)

on Bean

Photo by Sherrie Smith, University of Arkansas Cooperative Extension Service

Spider Mite Damage (Stippling)

on a Tree Leaf

Photo by Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Southern rust of corn is a destructive foliar disease caused by the fungus Puccinia polysora. Unlike common rust, which often appears earlier in the season and does not usually cause significant damage, southern rust can spread rapidly and cause substantial yield losses when it infects corn during critical growth stages such as during tasseling and silking. The disease does not overwinter in most of the United States but instead arrives each year via windborne spores from tropical and subtropical regions to the south.

Symptoms of southern rust appear as small, round-to-oval, orange or tan pustules concentrated mostly on the upper surfaces of leaves. These pustules are filled with powdery spores that are produced quickly (7 – 10 days after infection) and can easily be spread by wind. As the disease progresses, infected leaves may turn yellow, wither, and die prematurely. This loss of green leaf tissue reduces the plant's ability to photosynthesize, which can result in poor grain fill, reduced kernel weight, and overall yield loss. Unlike common rust, southern rust pustules are typically more densely packed and less likely to appear on the lower leaf surfaces.

Southern rust development is favored by high temperatures (80 – 90°F) and high humidity or extended periods of leaf wetness. In Arkansas, it commonly appears in late June to mid July, though timing can vary based on weather and the movement of spores from southern areas. Because the disease can develop and spread quickly under favorable conditions, regular scouting is critical once the crop approaches tasseling. Early detection allows for timely fungicide applications, which are most effective when made preventatively or at the very early stages of infection.

Effective management includes planting more tolerant hybrids when available and applying fungicides labeled for southern rust when disease pressure is high. Commercial growers should see pages 7 – 9 of the 2025 MP 154 Arkansas Plant Disease Control Products Guide for fungicide options. However, please read this note that is included at the top of page 7 of the products guide: “Fungicides should not be applied prior to 100% tassel and should not be applied later than 14 days after brown silk. We do not recommend the use of fungicides on field corn in Arkansas except under extraordinary circumstances. Preventative use will likely not result in an economic return on current hybrids grown in the state in most years. In rare cases where they might be needed – such as years favorable to southern rust, late-planted corn, or corn following corn in the same field – the following foliar fungicides are registered for use in Arkansas.” Cultural practices, like promoting good airflow in fields, may help reduce leaf wetness, though chemical control is often necessary in susceptible hybrids during high-risk years. For more information on southern rust of corn in Arkansas, please see this corn plant disease management article written by Dr. Travis Faske, UADA Extension Plant Pathologist, this corn disease identification and management video by Dr. Terry Spurlock, UADA Extension Plant Pathologist, and chapter 7, pages 7 – 8 of the MP437 Arkansas Corn Production Handbook by Dr. Travis Faske (UADA Extension Plant Pathologist) and Dr. Terry Kirkpatrick (Professor Emeritus University of Arkansas).

Southern Rust on a Corn Leaf

Photo by Edward Sikora, Auburn University, Bugwood.org

Urediniospores of Puccinia polysora, the Causal Agent of Southern Rust on Corn (200X Magnification)

Photo by Taylor Klass, University of Arkansas Cooperative Extension Service

Southern Rust on a Corn Leaf

Photo by Travis Faske, University of Arkansas - Division of Agriculture, Bugwood.org

Southern Rust on a Corn Leaf

Photo by Travis Faske, University of Arkansas - Division of Agriculture, Bugwood.org

Ash flower gall mite is a tiny eriophyid mite (Aceria fraxiniflora) that feeds specifically on the flowers of male ash trees (Fraxinus spp.). These mites are microscopic; they cannot be seen with the naked eye and are even smaller than spider mites, so they go unnoticed until their feeding activity causes the formation of conspicuous, abnormal growths known as galls. The galls start out green and fleshy in spring, often resembling small cauliflower-like clusters, and later turn brown, dry, and woody as the season progresses. While unsightly, ash flower galls do not typically harm the overall health of the tree.

The microscopic mites overwinter in bark crevices or near buds and become active in early spring when they migrate to developing male flower clusters. As they feed, they stimulate the plant tissues to grow abnormally, leading to gall formation. These galls may persist on the tree throughout the growing season and even into the following year, giving a cluttered, unattractive appearance to heavily infested trees. However, because the mites only target flowers and not leaves or stems, their impact on tree vigor is minimal.

Management of ash flower gall mites is rarely necessary from a plant health perspective but may be considered for aesthetic reasons. Since the mites are active very early in spring, any chemical treatment must be timed before bud break, which is difficult to predict precisely. Removing and discarding galled flower clusters can reduce the mite population over time but is generally impractical on large trees. For most situations, the best approach is simply to tolerate the cosmetic damage, as it does not threaten the long-term health of the ash tree.

Ash Flower Gall Mite Symptoms

Photo by Drew Johnson, University of Arkansas Cooperative Extension Service

Ash Flower Gall Mite Symptoms

Photo by Bruce Watt, University of Maine, Bugwood.org

This bulletin from the Cooperative Extension Plant Health Clinic (Plant Disease Clinic) is an electronic update about diseases and other problems observed in our lab each month. Input from everybody interested in plants is welcome and appreciated.

TAYLOR KLASS is a plant pathologist /instructor and JASON PAVEL is a plant pathologist, both with the University of Arkansas System Division of Agriculture. They are located at the Plant Health Clinic, 2601 N. Young

Avenue, Fayetteville, Arkansas.

Acknowledgements: Gratitude is due to Sherrie Smith, the originator of the Plant Health Clinic newsletters. Her works are a vital source of reference for the information provided herein.

"This work is supported by the Crop Protection and Pest Management Program [grant no. 2017-70006- 27279/project accession no. 1013890] from the USDA National Institute of Food and Agriculture."

University of Arkansas, United States Department of Agriculture and County Governments Cooperating

Pursuant to 7 CFR § 15.3, the University of Arkansas System Division of Agriculture offers all its Extension and Research programs and services (including employment) without regard to race, color, sex, national origin, religion, age, disability, marital or veteran status, genetic information, sexual preference, pregnancy, or any other legally protected status and is an equal opportunity institution.