By Erin Marissa Russell
Are you struggling with an outbreak of angular leaf spot, or will you be growing a vulnerable crop like cucumbers and want to know how you can make your garden safer against it? We’ve got everything you’ll need to diagnose when your plants have contracted angular leaf spot, to fight back against the disease if it’s taken hold in your garden, and to create a preventive treatment plan you can use to make sure your plants stay as healthy as possible. Keep reading to arm yourself against this disease.
Identifying Angular Leaf Spot
Angular leaf spot of cucurbit crops is a bacterial plant disease caused by the pathogen Pseudomonas syringae pv. lachrymans that is especially frequent when it rains often and for long periods of time. The disease is at its worst when the temperature stays between 75 and 82 degrees Fahrenheit (23 to 28 degrees Celsius).
While this strain of angular leaf spot strikes cucumber, honeydew, and zucchini the hardest, other plants it can affect include bryonopsis, cantaloupe, gourds of all varieties, muskmelon, pumpkins, squash crops of all kinds, vegetable marrow, watermelon, and West Indian gherkin.
Visual symptoms of angular leaf spot of cucurbits appear on the top side of plant foliage once infection sets in as small lesions with a water-soaked appearance. On the reverse side of leaves, the spots are shiny, with a gummy texture. Completely unfurled young leaves are more likely to show symptoms than older leaves. Edges may be either round or irregular, and the spots come in shades of white to gray. The lesions grow until they encounter the barrier of the leaf veins, creating the angular shape this disease is known for. As the disease progresses, the pale markings change color to brown, eventually drying out.
Once it has dried, the plant tissue shrivels, often causing parts of the leaf affected by angular leaf spot to fall out. This process is what causes the shothole pattern plants that have suffered from an angular leaf spot infection for a while without treatment often display. Once their transformation is complete, the plant’s once healthy leaves will have turned yellow and often become riddled with holes. When the weather is humid, the spots secrete a white discharge that dries on or next to the spots as a white crust.
On cucumber fruit, the lesions tend to always be round and will be smaller in size than the spots on the foliage. As the fruit matures, the spots eventually break open to expose a powdery white center. The wounds and openings in infected fruit provide a spot that provides a perfect opportunity for secondary infections and other pathogens to enter the plant. The tissue of cucumbers will eventually decay and develop a foul, rotten odor. All over the plant, a bacterial ooze may be emitted that becomes a white layer of crust once it is dry. Even after the fruit is harvested, new markings can still crop up while the cucumbers are being stored or transported.
How Angular Leaf Spot of Cucurbits is Transmitted
Angular leaf spot is spread via infected seed, so it is especially effective for gardeners to seek out certified disease-free seeds for planting. Once seeds germinate, the spread continues when water splashes over infected seedlings and onto disease-free crops that have already matured. Other forms of transmission include insects, gardening tools and equipment that are not sterilized between the infected plant and another plant or crop, and even from contact with hands and clothing of gardeners if those are not cleaned and sterilized. The soil around infected areas can also aid in the transmission of angular leaf spot if it is sandy and blown by the wind onto another, uninfected crop.
All these factors exist, but the plants and fruit cannot be infected unless water is present on their surfaces. The weather conditions that most facilitate spread of angular leaf spot include the following: ongoing rainy conditions, humidity at or exceeding 95 percent, soil that remains warm and wet, and low temperatures at night that give way to warmer daytime temperatures.
Weather also plays a role in stopping the spread of angular leaf spot. Two weeks of dry conditions will eliminate the potential for continued transmission. However, even five days during which the temperature exceeds 98 degrees will not have the same effect. Additionally, soil that contains an excess of nitrogen will increase the severity and duration of infections that already exist. Angular leaf spot also tends to appear during the early part of the season , and the vulnerable period extends through midseason.
Preventing and Treating Angular Leaf Spot of Cucurbits
Identifying the symptoms of angular leaf spot and recognizing the way that the disease spreads will aid in spotting the signs of illness. But there’s no reason to wait until a problem exists to begin working against angular leaf spot. Additionally, it’s important to know what to do once you have noticed the presence of this disease on your plants. Gardeners who practice preventive methods such as the ones outlined here are less likely to need the treatment methods you’ll also find in this section.
Practice Regular Crop Rotation
Crop rotation is one of the easiest and most important ways in which gardeners can work to actively prevent crop damage due to insect infestations and soil-borne plant diseases including angular leaf spot. Rotate where you grow your cucurbits, taking care to never grow them in the same field or garden bed in a three to four year span. Rotate your cucurbits out with other vegetable crops, fruits and ornamental flowering plants. Additionally, keep cucurbit crops out of areas where they might receive run-off water from adjacent fields where neighbors are growing cucurbit crops.
Whenever Available, Choose Certified Disease-Free Seed or Resistant Varieties
Many vulnerable crops (and especially cucumbers) have been cultivated to make disease-free seed options available to farmers and gardeners. When you start out with a prevention method such as certified disease-free seeds, the likelihood of needing to battle angular leaf spot in the fields or in your garden beds is drastically reduced. However, bear in mind that certified disease-free seed is not a completely reliable way to stop infection. Seeds that are labeled as certified disease-free have been shown in laboratory testing not to produce infected seedlings but are not guaranteed not to do so. There is also the option of resistant cucumber varieties. As opposed to the lab-tested certified disease-free seeds, resistant varieties have a genetic immunity or tolerance of angular leaf spot.
Eliminate Pest Insects From the Garden
Whenever insects infiltrate the flesh or foliage of a plant they are infesting, puncture wounds are created. Because these sites provide a handy point of entry for pathogens such as Pseudomonas syringae pv. Lachrymans, by reducing the amount of insects in the garden (at least pest insects as opposed to pollinators and other beneficial species), will also reduce the spread of angular leaf spot or, in uninfected crops the potential for infection.
Maintain A Dry Growing Environment
Though growing plants requires regular watering, soils should remain moist but never soggy during the growing season and care should be taken to avoid getting plants wet during the watering process. Try using drip irrigation techniques instead of overhead watering. If drip irrigation is not feasible for you, water your plants at the base, attempting to moisten only the soil below the plants, and keeping the plants as dry as possible to avoid soil-borne disease issues. Sometimes, getting your plants wet cannot be avoided, due to excessive rain and insufficient sunlight. Wait for your garden beds or crop fields to dry before cultivating, crop harvesting, or generally handling your plants when they are wet.
Avoid Fertilizing Plants with High-Nitrogen Feeds
Feeding your crops with excessive levels of nitrogen can intensify soil-borne disease issues. Avoid exacerbating conditions such as angular leaf spot by limiting the use of high-nitrogen fertilizers on your cucurbit crops.
Keep Your Grow Area Clean and Tidy
Between harvests, collect and remove all plant debris to lower the chances of harboring diseases. Fungal spores and bacteria hideout in plant debris between growing seasons and reinfect your crops. Immediately following harvests, collect crop debris by hand or cleanly plow beneath it and burn what you collect so that diseases cannot remain in the soil to reappear in future growing seasons.
Be Gentle When Harvesting Crops
Be careful not to damage your plants or your ripened fruit when harvesting by using a sharp, clean knife or garden shears to cut the cucumbers from the branch instead of ripping the fruit off the plants or pulling the stems off of the fruit. Use smooth, flexible harvesting containers to avoid bruising or damaging cucumber harvests, as bacteria can enter damaged fruits through open wounds.
Although angular leaf spot is extremely widespread to the point of being endemic in certain areas, gardeners and farmers do not need to simply accept the lost crops and income that an infection can result in. Although a bit of time and energy investment is required to implement a prevention plan using the methods above as well as to treat outbreaks that are already in process, doing so does not just lessen the losses associated with angular leaf spot for a single season.
Stamping out the disease will continue to have benefits by preventing the illness from returning again next season, and the sooner you take action, the less angular leaf spot will have the opportunity to spread among your plants. So don’t be disheartened by the signs and symptoms you may find in your fields or garden beds from time to time—armed with the techniques we’ve presented here, you know exactly how to handle angular leaf spot when it is in full force as well as how to prevent it from ever rearing its head in your garden.