Issue 5, June 24, 2025

Taylor Klass, Lead Diagnostician

Jason Pavel, Diagnostician

Cucurbit anthracnose is a fungal disease caused by Colletotrichum orbiculare that affects a wide range of cucurbit crops, including watermelon, cucumber, other melon, and bottle gourd crops. Pumpkin and squash crops can get cucurbit anthracnose but are less likely to be impacted by this disease. This disease is particularly problematic in warm, humid environments, where moisture facilitates rapid fungal growth and spread. The pathogen primarily targets the foliage, stems, and fruit, leading to significant yield loss and reduced fruit quality if not properly managed.

Symptoms of cucurbit anthracnose vary slightly by host plant but typically begin as small, water-soaked lesions on leaves. These lesions enlarge and turn brown or tan with a darker margin, sometimes becoming angular if restricted by leaf veins. Additionally, the middle of the lesions may crack, resulting in a “shot-hole” effect. On stems, petioles, and pedicles, the fungus causes lesions that are tan to dark, sunken, and spindle-shaped. These lesions can girdle and kill plant tissues and cause the abortion of immature fruit. Infected fruits often develop circular spots that can be either raised or sunken and may exhibit salmon-colored spore masses in humid conditions—a diagnostic sign of the disease.

The fungus survives between seasons in infected crop debris, seeds, and nearby weeds, making crop rotation and seed sanitation important cultural control methods. Wet weather, overhead irrigation, and dense canopies contribute to disease development by increasing leaf wetness duration, which is critical for spore germination and infection. Management strategies include using certified disease-free seed (do not save and use seed from infected plants/fruit), planting resistant cultivars where available, rotating out of cucurbits for several years, removing and destroying plant debris (or deep plowing debris), staying out of fields when they are wet, and applying fungicides preventatively or at the first sign of symptoms. Both commercial growers and home gardeners have multiple chemical products that they can choose from. Examples of products for commercial growers include chlorothalonil products (e.g. Bravo), thiophanate-methyl products (e.g. Topsin), and azoxystrobin products (e.g. Quadris). For more details and options, see page 143 of the 2025 Arkansas Plant Disease Control Products Guide. Examples of products for home gardeners include Ortho Garden Disease Control (chlorothalonil) and Bonide Mancozeb Flowable with Zinc Concentrate (mancozeb). For more details and options, including options labeled for organic use, see pages 156 and 157 of the 2025 Arkansas Plant Disease Control Products Guide.

Anthracnose Symptoms on a Watermelon Leaf

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

Anthracnose Symptoms on a Cucumber Leaf

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

Anthracnose Symptoms on a Cantaloupe Fruit

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

Anthracnose Spindle-Shaped Lesions on Watermelon Stems

Photo by Jason Brock, University of Georgia, Bugwood.org

Anthracnose Symptoms on

Cucumber Fruit

Photo by Charles Averre, North Carolina State University, Bugwood.org

Conidia (asexual spores) of Colletotrichum orbiculare, the Causal Agent of Anthracnose on Cucurbits

Photo by Rui map Zheng, Bugwood.org

Cucurbit gummy stem blight is a destructive fungal disease caused by several species of Stagonosporopsis (S. citrulli, S. cucurbitacearum, and S. caricae). It affects a variety of cucurbit crops, including watermelon, cantaloupe, cucumber, pumpkin, and squash. The disease is most severe during warm, wet conditions and can cause widespread damage to stems, leaves, and fruits, often leading to premature plant death and reduced yields.

Gummy stem blight first appears as small, dark brown or black spots on leaves, which may expand and develop concentric rings or become irregular and angular. These spots tend to start at the margins of the leaves before expanding further. On stems and vines, the fungus causes sunken, brown lesions that can exude a sticky, amber-colored sap—hence the name "gummy stem blight." However, be aware that other diseases and disorders can also cause stems to exude sap , so the presence of stem sap should not be used as the sole indicator of gummy stem blight disease. These lesions can girdle stems, leading to wilting and plant collapse. Fruit infection by Stagonosporopsis spp. is called black rot, and symptoms can vary widely based on the specific type of cucurbit but typically involve brown/black spots.

The fungus overwinters in infected crop debris and on seeds, and it spreads through splashing water, tools, and workers during cultivation. To manage gummy stem blight, it's essential to practice crop rotation with non-cucurbit crops for at least two years, avoid overhead irrigation, and remove and destroy infected plant material (or deep plow). Starting with certified disease-free seed, treating the seed (examples of seed treatments include heat or fungicides like 42-S Thiram Fungicide and Thiram 480 DP Seed Treatment Fungicide, which are registered for use in Arkansas), and applying fungicides early in the season can help limit disease spread. Monitor seedlings during transplant production and before planting in the field so that any symptomatic seedlings can be removed and destroyed. Regular scouting and early intervention are critical to managing this aggressive and often fast-spreading disease. Chemical products for managing gummy stem blight are similar to the chemical products for cucurbit anthracnose management. Examples of products for commercial growers include chlorothalonil products (e.g. Bravo), thiophanate-methyl products (e.g. Topsin), and azoxystrobin products (e.g. Quadris). Resistance to azoxystrobin and other FRAC code 11 products has become common in some fields. In such a case, Luna Experience has proven to be a good choice. For more details and options, see page 143 of the 2025 Arkansas Plant Disease Control Products Guide. Examples of products for home gardeners include Ortho Garden Disease Control (chlorothalonil) and Bonide Mancozeb Flowable with Zinc Concentrate (mancozeb). For more details and options, including options labeled for organic use, see pages 156 and 157 of the 2025 Arkansas Plant Disease Control Products Guide.

Gummy Stem Blight Symptoms on a Watermelon Leaf

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

Gummy Stem Blight Symptoms on a Cantaloupe Leaf

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

Black Rot of Squash

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

Fruiting Bodies of Gummy Stem Blight on a Watermelon Stem Lesion

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

Black Rot of Watermelon

Photo by Jason Brock, University of Georgia, Bugwood.org



Impact of Gummy Stem Blight

in the Field

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

Pycnidia (asexual fruiting body) of Stagonosporopsis Exuding a Gelatinous Mass of Conidia (asexual spores) at 20X Magnification

Photo by Jason Pavel, University of Arkansas Cooperative Extension Service

Bacterial spot of tomato and pepper is caused by several closely related species of bacteria in the genus Xanthomonas. This disease is a major concern in warm, humid climates and can cause severe yield losses by damaging leaves, stems, and fruit. The pathogen spreads rapidly under favorable conditions, particularly through splashing water, wind-driven rain, contaminated tools, and infected seed, transplants, or nearby weeds. We have been seeing a lot of bacterial spot in the clinic recently as a result of all the rain we have had! 

Symptoms on tomato and pepper plants typically begin as small, water-soaked spots on leaves, which gradually enlarge and become dark brown. The spots may be surrounded by yellow halos. Infected leaves may become tattered and drop prematurely, reducing the plant’s photosynthetic capacity. If you touch a pepper leaf infected with bacterial spot, that leaf will easily fall off the plant. Xanthomonas bacteria can also cause oval, dark brown spots on tomato and pepper stems and petioles. On fruit, bacterial spot results in raised, scabby lesions that make the produce unmarketable, although the lesions don’t usually penetrate deeply into the fruit. These fruit spots are particularly damaging for commercial production of fresh market tomatoes due to aesthetic quality standards. Bacterial spot disease can be easily confused with several other diseases of tomato and/or pepper, such as bacterial speck (caused by a bacterium in a separate genus, Pseudomonas), Septoria leaf spot (caused by a fungus), gray leaf spot (caused by a fungus), and tomato spotted wilt (caused by a virus). Therefore, an accurate diagnosis of the disease-causing pathogen is key for correct and effective disease management. 

Xanthomonas bacteria survive between seasons in crop debris, volunteer plants, weeds, and contaminated seeds. Control of bacterial spot relies heavily on prevention, as there are limited effective treatments once symptoms appear. Strategies include planting disease-free, certified seed and resistant varieties where available (currently available only for peppers), practicing crop rotation, removing infected plant material and nearby weeds, and minimizing leaf wetness through drip irrigation and good spacing. Copper-based bactericides, often combined with mancozeb, may help prevent initial infection and spread, but resistance is common, and efficacy varies. Commercial growers should also consider incorporating Actigard 50WG into their repertoire for bacterial spot prevention. Early detection and integrated management of bacterial spot disease are essential to prevent outbreaks and protect yields.

Bacterial Spot Symptoms

on a Tomato Leaf

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

Bacterial Spot Symptoms

on Tomato Stems

Photo by Florida Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Bugwood.org

Bacterial Spot Symptoms

on Tomato Fruit

Photo by Sherrie Smith, University of Arkansas Cooperative Extension Service

Bacterial Spot Symptoms

on Pepper Leaves

Photo by Sherrie Smith, University of Arkansas Cooperative Extension Service

Oak leaf blister, caused by the fungal pathogen Taphrina caerulescens, is a common disease of oak trees. The fungus infects newly emerging leaves in the spring, leading to noticeable symptoms as leaves expand. Infected leaf tissue becomes distorted, forming raised, blister-like areas on the upper surface and corresponding sunken patches on the lower side. These blisters are typically light green at first, later turning yellow and brown as the infected tissue dies. A similar disease, peach leaf curl, can occur on peach leaves and is also caused by a fungus in the Taphrina genus. 

The infection process begins in early spring when ascospores of Taphrina land on young, unfolding leaves during cool, wet weather. The fungus grows between leaf cells, stimulating abnormal cell division and expansion, which cause the characteristic blisters. The pathogen does not penetrate deeply into the leaf tissue and tends to remain localized, meaning it rarely threatens the overall health of the tree. However, heavy infections can cause premature leaf drop and mild aesthetic damage, which may be concerning in ornamental settings.

Environmental conditions play a critical role in the development of oak leaf blister. Cool (50–70°F), wet springs with prolonged leaf wetness are ideal for infection. Once leaves mature, they become resistant to infection, and the fungus ceases to be active. The fungus overwinters on twigs and bud scales of oak.

In most cases, chemical control is unnecessary, but in situations where the disease is severe and impacts tree aesthetics or vigor year after year, a dormant fungicide application (such as chlorothalonil, copper, or mancozeb products) in late winter or early spring before bud break may reduce infection. Fungicides applied after bud swell do not have any effect. Maintaining overall tree health helps trees tolerate and recover from infections. Making sure even adult trees are thoroughly watered around once a week during the summer dry season can be especially useful in reducing tree stress and maintaining overall tree health. Protecting bark from extreme temperature changes in the winter, testing soil for pH and nutrient composition every two years, and protecting young trees from sunburn are other measures that can help maintain overall tree health.

Symptoms of Oak Leaf Blister

on an Oak Leaf

Photo by Sherrie Smith, University of Arkansas Cooperative Extension Service

Symptoms of Oak Leaf Blister

on an Oak Leaf

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.