Identifying Stone Fruit Bacterial Diseases

Stone fruit diseases are many and varied, but they typically come from four main origins:

• Phytoplasmas
• Fungus
• Bacteria
• Unknown origin

Knowing which kind of disease you’re dealing with can help you identify effective methods of prevention and control to protect your fruit and avoid a loss. Here, we’ll take a look at the most common bacterial stone fruit diseases (including those that affect almonds), together with common symptoms and tips for identification.

Bacterial Canker

Bacterial canker is one of the most widespread stone fruit diseases in the world, spanning North America, Europe, South Africa, and unconfirmed reports from Lebanon and Australia. Many California growers start worrying about bacterial canker over the period of winter and bloom.

This disease is caused by the bacteria Pseudomonas syringae pv. morsprunorum and often occurs together with P. syringae pv. syringae. Killing buds, fruiting spurs, branches, and trees, bacterial canker can cause significant damage to cherry, plum, and prune trees. You can spot bacterial canker with the following signs and symptoms:

• Blossom and spur blight seen after bloom
• Fluorescent yellow coloring on leaves
• Angular necrotic spots on leaves
• Leaves become tattered and fall
• Cherries display deep, black depressions on the fruit and stem
• Depressed cankers at the base of blighted buds, spurs, and scaffold branches
• Shoots and branches that die back from expanding cankers

For laboratory testing, the disease can be isolated using King’s B medium.

Bacterial Leaf Spot

Bacterial leaf spot is caused by the gram-negative bacterium Xanthomonas campestris pv. pruni, and affects almond, cherry, peach, plum, and apricot orchards. While this stone fruit disease is distributed in countries all around the world, it was only confirmed in the central regions of the San Joaquin Valley as recently as 2014.

In contrast to bacterial canker, bacterial leaf spot affects the leaves, fruits, and twigs (rather than blighting the buds) in the rainy season and can appear as late as the end of summer.

Look for the following symptoms to identify bacterial leaf spot:

• Small irregularly-shaped spots on leaves
• Yellow, purple, and brown leaf discoloration
• Shot-hole appearance where the spots have dropped out
• Small brown spots on fruit with dark-green wet halos
• Sunken circular lesions on twigs
• Sunken elliptical lesions or dark purple cankers in summer

Bacterial leaf spot can be confirmed with a detached-leaf bioassay. In a selective medium, the water-soluble yellow coloring should be clearly evident.

Crown Gall

Like the two stone fruit diseases that we’ve described, crown gall is also very widespread and affects Prunus spp. as well as roses, small fruits from the Rubus spp., pome fruits, and dicotyledons. Caused by the gram-negative bacterium Agrobacterium turnefaciens, crown gall affects plants primarily in nurseries, rendering them unable to be sold or exported.

The main symptom of crown gall is the appearance of knob-like galls on all of the woody parts of the plant — especially on the roots, graft unions, and close to the soil surface (known as the “crown”).

To confirm this bacterial disease, the sample can be cultivated on indicator plants, including tomato, sunflower, Datura, and Bryophyllum, or identified at a molecular level with a PCR test.

Phony Peach, Plum Leaf Scald, Almond Leaf Scorch

These three stone fruit diseases are caused by the same pathogen — a gram-negative bacteria called Xylella fastidiosa. However, they express themselves as different pathotypes depending on which plant they infect. Pierce’s disease causes almond leaf scorch specifically, whereas phony peach and plum leaf scald are caused by a different strain of Xylella fastidiosa.

The symptoms of phony peach are:

• Reduced tree and fruit size
• Flattened, umbrella-like shape
• Dense, dark-green foliage
• Unusually short stem internodes

The main symptom of plum leaf scald and almond leaf scorch is marginal leaf scorching, although host trees can carry these stone fruit diseases without presenting symptoms.

All three of these stone fruit diseases can be detected with phase contrast, fluorescence microscopy, ELISA, PCR testing, and culturing on selective media. These diseases more commonly occur in moderate climates because the bacteria are sensitive to cold temperatures. According to a study from 1973 (Goheen et al.), the dormant wood can be treated with hot water at 45°C for 3 hours.

Protect Your Crops with Fruit Growers Supply

Whether you’re looking to start a new orchard, prevent a recurring disease, or treat stone fruit diseases that are affecting your orchard, make Fruit Growers Supply Company your one-stop-shop for information, crop-specific irrigation design, specialty sprays, and on-site consultations. You can reach us by phone at 866-477-7414 or drop by one of our retail stores in Porterville, Orange Cove, Woodlake, Santa Paula, and Riverside. Stop stone fruit diseases from taking hold and ensure a healthy stone fruit harvest.