Like the all-too-common disease bacterial canker, citrus blast (sometimes called bacterial blast) is caused by the bacteria Pseudomonas syringae. Symptoms of citrus blast first become evident after a period of wet, windy weather, usually in the winter or early spring. The first symptoms are dark, reddish-black spots on the petiole—the short stem running between leaf and stem. Eventually trees develop large areas of reddish-black scabbing, and the disease spreads to the fruit, where it causes black spotting.
The first signs of citrus blast appear in the form of black, watery lesions on the petioles of newly grown leaves.
From here, the disease spreads out to the mid-vein of the leaf and back to the axil, the joint between the petiole and stem. Symptoms may also initially appear on the tips of young leaves and shoots.
As leaf petioles are encircled (girdled) by the lesions, the leaves turn yellow, wither, dry out and die. Dead leaves will stay firmly attached for days or weeks, but eventually drop away from the tree, leaving the petioles attached. Younger twigs experience a similar process, with black, necrotic cankers forming on and spreading across their surfaces. Infected twigs typically die within a few weeks. However, while the disease can spread through new and old growth, newer growth is much more susceptible to the ravages of the disease. Older growth is more resilient and may produce some lesions, but have enough resistance to not allow infections to girdle tissues and cause dieback.
The path of the disease is easy to trace through the tree. The pathogens spread from the initial infection sites to adjacent twigs and limbs, and throughout the tree. As the disease spreads, a reddish-brown scab appears on the surfaces of infected leaves and twigs. These scabby areas may become sunken and exude a reddish gum or sap, which can spread the disease.
It should be noted that the spread of P. syringae is confined to surface tissues. Even badly infected branches will often not show signs of infection within or below the cambium layer. This contributes to the fact that older growth can better survive the disease.
Eventually, the disease will spread to the fruit, where it causes small black spots or pits on the skin.
Citrus blast is caused by Pseudomonas syringae, an opportunistic bacterium which gains access into plant tissue via surface damage.
This pathogen is unable to penetrate the surfaces of plant tissues on its own. Instead, it relies on assistance from insects, field hands, and other helpful parties to damage plants and create a literal and figurative opening for the bacteria to enter through. However, P. syringae’s preference for wet, cool conditions aligns with the windy weather that occasionally occurs in the winter and spring.
Consequently, citrus blast often gains a foothold in orchards in the aftermath of windy rainstorms and hailstorms Symptoms will first appear on the sides of trees that face the direction the wind blows from—in most cases, the south side.
In California’s Central Valley, the citrus species most vulnerable to citrus blast are orange, grapefruit, and lemon. Orange and grapefruit tend to contract the disease via damage to leaves and twigs, while on lemon trees, the bacteria often enters through the fruit.
There are chemical treatments available for helping to prevent the onset of citrus blast, but treatment of infections typically relies on careful sanitation.
Copper sprays—either Bordeaux or fixed copper—have long been applied to plants before the rainy season to reduce the likelihood of bacterial or fungal infections. Such sprays have shown some effectiveness in preventing or reducing the severity of citrus blast.
However, such sprays are not bulletproof. Growers are advised to take other steps to improve the hardiness of the crops. Small twigs and rapid growth are especially vulnerable to the types of damage that allow P. syringae to invade. Thus, these types of growth should be pruned back in the fall, and growers should avoid heavy use of fertilizers in the fall and winter, especially nitrogen-rich fertilizers which promote rapid growth with low structural integrity. Incorporating products such as Integrity 9.5% Calcium and Integrity FG-Micros into your feeding regimen will help to promote the growth of hardier, more sustainable growth that is less vulnerable to damage and infection by P. syringae.
If an outbreak of citrus blast occurs, any diseased leaves or twigs should be pruned away and removed from orchards immediately. Do not allow diseased tissue to remain on the ground. Pruning tools should be sanitized with alcohol or Lysol between prunings to avoid spreading the disease.
(Thumbnail image sourced from Pxhere, licensed via the public domain.)