Leaf Spot

It’s not unusual to see a spot on a leaf. No big deal, right? Maybe.

A single spot on a leaf might be nothing, or it can be home to more fungal spores than we can count. Each of those spores can catch a ride on a breeze or a raindrop to infect nearby leaves and plants. Before you know it, your garden has a fungal problem.

Septoria leaf spot on tomato (Dr Parthasarathy Seethapathy, TNAU, bugwood.org) CC BY 3.0

Symptoms of leaf spot

Symptoms can be insignificant spots, solitary bumps, or entire patches of brown, black, purple, or yellow over most of a leaf. You can determine what causes leaf spot by considering the plant species affected. Once you identify the cause of leaf spots in your garden, you will be better able to help your plants.

Alternaria leaf spot

The cabbage family, escarole, mango, calendula, and marigolds are susceptible to Alternaria leaf spot. Also known as Alternaria leaf blight, this disease starts out looking like other leaf spot diseases. But it can become a bigger problem, distorting and damaging broccoli and cauliflower heads.

Bacterial leaf spot in almonds

Bacterial spot (Xanthomonas arboricola pruni) is a relatively new disease that can severely reduce your almond crop. Symptoms begin in spring when dark spots develop where water collects, turning yellow and causing premature leaf drop. This bacteria also attacks the nuts, causing reddish lesions that later ooze an amber-colored gum. The nut inside shrivels up and becomes inedible. Lesions and cankers also occur on young twigs. Remove all infested leaves, cut away infected wood, and remove mummies. You can also interrupt this bacterium’s lifecycle by removing all the leaves before the rainy season hits. Fixed copper sprays in late January and again in late March can protect your almond trees from future infestations.

Bacterial leaf spot in spinach

Spinach leaves with water-soaked, angular areas of damage probably have bacterial leaf spot (Pseudomonas syringae spinaciae). What starts as tiny dark spots grow into large (3/4”) angular damaged areas that can merge with nearby areas for complete leaf loss. Infestations are visible on both the top and the bottom of the leaf. Overhead watering is the most common cause of bacterial diseases.

Common leaf spot in cane fruits

Your blackberry or raspberry plants may play host to the Mycosphaerella rubi fungus. Common leaf spot can also appear in gooseberry, boysenberry, dewberry, and olallieberry. The leaf spots are brown and circular with dark brown or purple margins and white centers. Oval lesions also occur on canes. Improved air circulation is the best treatment.  Copper and sulfur sprays may also provide some control.

Common leaf spot in strawberries

​Strawberries are vulnerable to a fungal disease called common leaf spot (Ramularia tulasneii). Symptoms include tiny purple dots on the upper leaf surfaces. These dots can grow to 1/4” in diameter, turning brown, gray, and white. Lesions may form on stems (petioles), stolons  (runners), and fruit clusters (trusses). The lesions block the flow of water and nutrients in the vascular bundles. Common leaf spot is the most destructive leaf spot disease of California strawberries. Commercial growers use a fungicide called chlorothalonil (Bravo, Echo, and Daconil) on plants in the nursery to eliminate this pathogen. The use of resistant cultivars is the only option available to home growers.

Entomosporium leaf spot fungus in pomes

Apples, crabapples, loquat, pears, and quince are all vulnerable to the Entomosporium leaf spot fungus (Entomosporium mespili, aka Diplocarpon mesophilic). Symptoms of Entomosporium leaf spot include reddish spots with dark purple, red, or yellow halos, pale glossy specks in the center of these lesions, and early leaf drop. Fixed copper sprays can help reduce the infection.

Septoria leaf spot in tomatoes

Septoria leaf spot affects celery, chicory, cucumber, other cucurbits, parsley, and tomatoes. Plants infected with Septoria develop tiny yellow spots on leaves. These may be angular or round, with a reddish or yellow halo. These spots eventually merge and turn brown, killing leaves. Remove infected plants and throw them in the trash.​

Leaf spot controls

Since fungicides are not considered justifiable for most causes of leaf spots, these cultural controls are your best bet:

• Collect and dispose of diseased leaves and other plant materials.
• Do not add diseased plants to the  compost pile.
• Use resistant plant varieties.
• Avoid overhead watering.
• Prune plants for better airflow.
• Avoid using excessive amounts of fertilizer.