Angular Leaf Spot
Author: Howard F. Schwartz and David H. Gent
(Smith and Bryan 1915) Young et al. 1978
Identification and Life Cycle
Angular leaf spot of cucurbits is caused by the bacterium Pseudomonas syrinage pv. lachrymans, and is the most common bacterial disease of cucurbits in the world. The disease cycle begins when seedborne inoculum or bacterial cells in infested crop debris colonize cotyledons upon germination. The bacterium infects hosts through natural openings and wounds, multiplying in and on leaves. Windblown soil containing infested crop debris is an effective means of spreading the bacterium and disease. The pathogen can be disseminated within and among fields by irrigation water, splashing rain, insects, workers, contaminated equipment, and by wind as aerosols. P. syrinage pv. lachrymans survives betweens cucurbit crops in contaminated seed and infested crop debris.
Plant Response and Damage
Angular leaf spot symptoms appear as small, water-soaked lesions on leaves, and are largely confined by large plant veins. Lesions may or may not have yellow margins. Lesions often have an angular appearance because they do cross plant veins. During humid weather a clear to milky exudates is apparent on the lower surface of the lesion. This exudate becomes a white crust as it dries. Lesions become dry as they age, turning a tan brown and sometimes dropping out of the leaf. Determinate types of cucurbits are often damaged more seriously than indeterminate types because growing tips are sometimes infected systemically and cause vines to stop growing. Lesions can also develop on petioles, stems, and fruits. A thin, whit crust is present on all infected plant parts. Fruit lesions are small (0.04 to 0.1 inches in diameter) and water-soaked, often with a light tan center. Fruit infection can also penetrate deeply, causing an internal rot of vascular tissue of the fruit and sometimes deformation of the fruit. Secondary pathogens often attack infected fruit. Angular leaf spot can reduce fruit yield, quality, and marketability.
No biological control practices have been developed for angular leaf spot.
Planting pathogen-free seed is the primary control strategy for angular leaf spot. Contaminated seed can be treated with hot water to reduce, but not eliminate, the number of bacteria in and on the seed. Practice a three-year or longer crop rotation to nonhosts such as small grains or corn. Avoid overhead irrigation and reuse of irrigation tail water, if possible. Only cultivate and work in fields when the canopy is dry to reduce secondary spread of the pathogen. Many cucumber varieties and other cucurbits are resistant to angular leaf spot and should be planted if the disease becomes a recurrent problem in cucurbit production.
Copper-based bactericides are often necessary to reduce the severity of angular leaf spot in warm, humid production regions. A four to seven day spray interval is often necessary when conditions are highly favorable for disease development. Chemical controls are most effective when integrated with sound cultural control practices.
Product List for Angular Leaf Spot:
|Pesticide||Product per acre||Application Frequency (days)||Remarks|
|Actigard 50WG||0.5-1 oz||7 days||Maximum of 8 oz to non-stressed plants; 0 day PHI|
|Badge||16-40 oz||7 days|
|Champ Dry Prill||1.33 lb||5-7 days||May cause injury|
|Champ Formula 2||1.33 pt||5-7 days||May cause injury|
|Copper-Count-N||4-6 pt||7 days||May cause injury|
|Kocide 101||1.5-3 lbs||5-7 days||May cause injury|
|Kocide DF||1.5-3 lbs||5-7 days||May cause injury|
|Kocide 4.5LF||1-2 pts||5-7 days||May cause injury|
|Kocide 3000||0.5-1.25 lb||5-7 days||May cause injury|
|Nordox||1.5-2.0 lb||5-7 days||May cause injury|
|Tri Basic Copper||2-4 pt||5-7 days||May cause injury|
|Cuprofix MZ Disperss||4-7.25 lb||3-10 days||Maximum of 63.1 pounds per season; 5 days PHI|
|ManKocide||2.0-2.5 lb||7-10 days||Maximum of 128 pounds per season; 5 days PHI|
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.