Blister Spot: An Emerging Disease in Commercial Apple Orchards in Illinois 

Blister spot of apple is caused by the bacterium Pseudomonas syringe pv. papulans. The reports prior to 2000 show that at least 25 apple cultivars were susceptible to blister spot. Most of those cultivars are no longer in commercial production. Since the turn of the twenty-first century, the disease has mainly been of economical concern on the Mutsu (Crispin) cultivar. In the past two decades, no blister spot was noticed in Illinois. In 2024, however, sever blister spot was observed on Fuji cultivar in one commercial orchard in southwest of the state. Then, a quick survey of the commercial orchards in 2024 showed that blister spot occurred on ‘Matsu’ and ‘Fuji’ apples in five commercial orchards in the western and southern parts of Illinois.

Purplish black lesions develop on fruits. Symptoms are first noticeable 2-3 months after petal fall as small, green, water-soaked raised lesions that develop on fruits surfaces (Figures 1 and 2). As the growing season progresses, the lesions expand to 4-5 mm in diameter and become darkened (Figure 2). More than 100 lesions may develop on a single fruit.

P. syringe pv. papulans overwinters in buds and leaf scars and may also overwinter on diseased fruits on the orchard floor. Throughout the growing season, P. syringe pv. papulans may survive as an epiphyte on foliage and fruit and on weeds in the orchard. As symptoms become visible, masses of bacteria associated with fruit can be observed using microscope. The pathogen may be spread in water, and blister spot development appears to be favored by wet weather.

Streptomycin applications have been reported to be most effective in managing blister spot. The first spray is applied 10-14 days after petal fall, followed by two sprays at weekly intervals. Strains of the pathogen resistance to streptomycin have been reported in other apple growing areas. Other antibiotics and copper materials have been tested for blister spot control, with only moderate success.

This is an emerging disease in Illinois. If the disease becomes an economically important problem, efficacy of potential compounds for managing blister spot should be investigated.

Mohammad Babadoost

Professor of Plant Pathology and Extension Specialist

Department of Crop Sciences

UIUC