Better Yields Through Science

Plum Diseases & Pests

DiseaseSymptoms, Life-cycle and Pest Management
Panicle and Shoot BlightPathogen: Botryosphaeria dothidea; conidial stage: Fusicoccum sp.

Vegetative and flowering buds that were killed during the previous fall or winter do not emerge in spring. In mid-spring (end of May to June) buds that were partially infected the previous season produce fruit clusters and shoots that develop blight from the fungus in buds. The rachises of these blighted clusters turn black as do the shoots. When temperatures increase in May through July, the fungus moves into shoots of the previous year, causing blighting of fully developed clusters. These blighted shoots, leaves, and clusters turn brown.

Sources of inoculum for this disease are rachises, shoots, and petioles killed during the previous growing season that remain on the trees. Cankers can also provide inoculum for as long as 6 years. Spores from these sources cause primary infections on the vegetative and flowering buds.
Alternaria Late BlightPathogens: Alternaria alternata, A. tenuissima, A. arborescens, and Stemphyllium spp.

Alternaria late blight occurs as black angular or circular lesions on leaves of both male and female trees. When the pistachio fruit are maturing, black lesions with a red halo appear on hulls. Black spores develop in the center of the leaf lesions when humid conditions prevail in orchards. Rubbing the leaf lesions with a finger will blacken the finger. In contrast, rubbing a finger on a lesion cause by Botryosphaeria dothidea does not blacken fingers because there are no spores produced on the surface of these lesions. However, late in the season both fungi can be present in the same lesion and microscopic identification will be needed. Leaf infections can cause severe premature defoliation and create problems during shaking the trees at harvest.

Black lesions are also present on petioles and main veins of leaf blades. On immature fruit the symptoms appear as small black lesions about 1 mm in diameter. On mature fruit, both small (1–2 mm) and large (5 mm) black lesions are present on the epicarp, usually surrounded by a reddish purple margin. Multiple lesions on leaves and fruit cause leaf blight and deterioration of hulls, respectively. Deterioration of hulls results in shell staining. Orchards with cover crops have more Alternaria blight than disked orchards. Sunburned fruit become more susceptible to Alternaria blight than non-sunburned fruit.