Symptom Identification and Control Methods for Azalea Leaf Gall

Symptoms:

Azalea leaf gall, also known as cookie disease. The young tissues of azaleas can all be affected, mainly damaging flower buds, young leaves and new shoots. On the upper surface of the leaves, initially pale yellow semi-transparent round spots appear, later turning yellow and sunken; the underside of the leaves is pale red, thickened and swollen, subsequently raised into galls, with a thick grayish-white powdery layer on the gall surface, like a cookie, and the leaves wither, yellow and fall early. In severe cases, the petiole lesions merge into patches, deformed and thickened. When young shoots are affected, fleshy lotus-like leaves or tumor-like leaves are produced at the tips, later shrinking into sac-like forms. After the petals are infected, they become abnormally thickened, forming irregular galls. After the flower buds are affected, they become fleshy, thickened and hardened. After these young leaves and flower buds are infected, they abnormally become fleshy and thickened, producing fungal galls of varying sizes, which flower growers call "azalea apples," with a diameter of 3-12 mm, some reaching 8-18 mm, covered on the surface with a grayish-white powdery substance, which is the basidia and basidiospores of the pathogen. Finally, the diseased parts turn dark brown, dry up and fall off.

Pathogen and disease occurrence patterns:

Fungus. It is the basidiomycete Exobasidium japonicum Shirai. or Exobasidium rhododendri Cram. Phylum Basidiomycota, class Hymenomycetes, order Exobasidiales, family Exobasidiaceae, genus Exobasidium.

Late spring and early summer and late summer and early autumn are the peak periods of disease occurrence. The basidia of the pathogen form beneath the epidermal tissue of the azalea host. After maturity, they break through the epidermis and become exposed, bearing cylindrical basidiospores on top, by which they infect and spread. Low temperature and high humidity make the disease prone to occur. A monthly average temperature of 15-20°C and a relative humidity above 80% favor disease occurrence. The azalea leaf gall pathogen overwinters as mycelium latent within the plant tissues. In the following spring, basidiospores are produced and spread by wind and rain splash, infecting young tissues. The incubation period is 7-17 days. The disease occurs twice a year, in late spring and early summer and in late autumn and early winter, with late spring and early summer being the most common, and March to May being more severe. When the temperature is relatively low, with a monthly average temperature of 15-20°C, relative humidity above 80%, continuous rain, insufficient sunlight, and tender plant growth, the disease is prone to become epidemic. Alpine rhododendrons are the most severely affected.

Control methods:

(1) When symptoms first appear and before white powdery substances have formed on the diseased parts, immediately remove the infected tissues and burn them in a centralized manner. Do not introduce diseased seedlings into disease-free areas.

(2) Before bud break, spray 2-5 degrees Baumé lime sulfur mixture.

(3) Before disease onset, especially during shoot emergence and leaf expansion, spray 0.5-1% Bordeaux mixture or 80% zineb 500-fold solution. At disease onset, spray 65% zineb 500-fold solution, or 0.3-0.5 degrees Baumé lime sulfur mixture 3-5 times.