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, or it may also indicate anthracnose. Each of those spores can catch a ride on a breeze or a rain drop 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 range from insignificant, solitary dots or bumps to entire patches of brown, black, purple, or yellow that cover the majority of a leaf. It is easiest to determine what causes leaf spot by considering the type of plant being infected. Once you identify the cause of leaf spot in your garden, you will be better able to help your plants.


​Almonds

​A relatively new disease, Bacterial spot (Xanthomonas arboricola pruni) 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 themselves, causing reddish lesions that later ooze an amber-colored gum. The nut inside shrivels up becomes inedible. Lesions and cankers may also be seen 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. (That’s what I did.) Copper sprays in late January and again in late March can protect your almond trees from future infestations.


​Apples & pears

Members of the pome family include apple, crabapple, roses, loquat, pears, quince, pyracantha, and toyon. This group of plants is frequently infected by the Entomosporium leaf spot fungus (Entomosporium mespili, aka Diplocarpon mespili). This fungus shows as reddish spots with a dark purple, red, or yellow halo, and leaf drop is common. As the fungi grow, light-colored glossy specks form in the center of these lesions. These specks contain new fungal spores. Copper fungicides can help reduce the infestation.


​Black Spot

Black spot on roses is caused by a fungus (Diplocarpon rosae). If you look closely, or use a hand lens, you might be able to see the feathery surface of these fungal growths, which are only found on the upper side of leaves, and not underneath. Overhead watering and poor airflow are the most common causes of black spot.


Cabbage family, escarole, mango, calendula and marigolds

This odd assortment of plants is all susceptible to Alternaria leaf spot. Also known as Alternaria leaf blight, this disease starts out looking like other forms of leaf spot, but can grow into a bigger problem, distorting and damaging broccoli and cauliflower heads. There are several types of Alternaria fungi that cause leaf spotting. The ones that attack members of the Brassica family are Alternaria brassicae and A. brassicicola.

​Caneberries

Your blackberry or raspberry plants may play host to the Mycosphaerella rubi fungus. Gooseberry, boysenberry, dewberry, and olallieberry may also be infected. These leaf spots are brown, circular, with dark brown or purple edges, and white centers. Oval lesions can also be seen on canes. Improved air circulation is the best treatment. Copper and sulfur sprays may also provide some control. This condition is also known as common leaf spot.


​Lawns
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Blades of grass are susceptible to gray leaf spot (Pyriculria grisea) and leaf spot caused by the Bipolaris and Helminthosporium subspecies. The latter infestations look like round or oval brown spots with purple borders on stems, blades, and sheaths. Crowns and roots can also become infected and die. Grey leaf spot symptoms are just the opposite: the spots look bleached, with a brown border. Infested ryegrass can develop a fishhook shape. Avoid the use of excessive nitrogen, which stimulates vulnerable new growth and be sure to irrigate as early in the day as possible, to give plants time to dry out. Aerating the soil and taller mowing height can also help.


Ornamentals

Poplar, cottonwood, azalea, snapdragon, and hebe, are just a few plants infected by the family of Septoria fungi. Symptoms include light brown leaf spots with dark brown halos,  partial leaf die off, and twig cankers and lesions. This is an internal pest, so infected wood must be removed.


​Spinach

Spinach leaves with water-soaked, angular areas of damage are probably infected with bacterial leaf spot (Pseudomonas syringae spinaciae). What starts as tiny dark spots grows to large (3/4”) angular damaged areas that can merge with nearby areas for complete leaf loss. Infestations can be seen from both the top and the bottom of the leaf. This bacterial disease is almost exclusively caused by overhead watering.


Strawberries

​Strawberries are frequently attacked by a fungi called Ramularia tulasneii. Symptoms include tiny purple dots on the upper side of leaves. These dots can grow to 1/4” in diameter, turning brown, grey, and then white. Lesions may form on stems (petioles), stolons (runners), and fruit clusters (trusses) that block the flow of water and nutrients in the vascular bundles. This 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, but the use of resistant cultivars is really the only option available to home growers. Strawberries are also vulnerable to common leaf spot.


Leaf spot controls
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Since fungicides are not considered justifiable for most causes of leaf spot, good sanitation and cultural controls are your best bet. This means:

• 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 air flow.
• Avoid using excessive amounts of fertilizer.

Cabbage family, escarole, mango, calendula and marigolds


This odd assortment of plants is all susceptible to Alternaria leaf spot. Also known as Alternaria leaf blight, this disease starts out looking like other forms of leaf spot, but can grow into a bigger problem, distorting and damaging broccoli and cauliflower heads. There are several types of Alternaria fungi that cause leaf spotting. The ones that attack members of the Brassica family are Alternaria brassicae and A. brassicicola.