Apple ash (Podosphaera leucotricha)

Powdery mildew is a widespread disease in orchards, which severely affects several sensitive apple varieties cultivated in bright and low-humidity locations, such as Jonathan, Idaredi, Boscopi, Delishes, and Strakingi. This disease affects buds, flowers, leaves, and seedlings, although most varieties are immune to it.

 

 

Apple powdery mildew appears on leaves in the so-called 'mouse ears' stage, but it is more noticeable on flowers and young shoots. This disease affects bud opening, as infected buds later bloom. Infected leaves curl, forming a powder layer of spores with a bitter color. Infected buds are covered with fungal mycelium, which causes the buds to be noticeably smaller in size. Secondary infections occur on leaves, which are covered with a mycelium layer and take on a bitter color, especially on the lower parts of the leaf.

 

 

Phytopathogenic fungi overwinter in infected apple buds, in the form of mycelium or cleistothecia. On infected plant surfaces, mycelium forms egg-shaped or round conidia. These organs germinate at an average temperature of 18-25 °C and at relative air humidity above 35%. Generative reproductive organs (cleistothecias) form in infected parts and have an almost round shape. They are equipped with several extensions.

The fungus also overwinters in the form of cleistothecs, which form at the end of the vegetation on plant parts on fallen leaves and on seedlings. It is characteristic that conidia can germinate without the presence of free water and at relatively high humidity of 80-90%.

The largest number of these organs form during the months of May and June, at temperatures around 20°C. If there is less atmospheric precipitation during the year, then they form on a larger scale. It should be mentioned that the more stable varieties are golden curlysh, red delishes, etc.

 

 

Protection against the disease begins when the first signs appear on the branches, and where powdery mildew has overwintered, pruning should be carried out to reduce the risk of infections. Control starts in the phase before the pink color of the buds appears, because the mycelium can be active early, already during the growing season. Fruit infection appears at the bud stage, with a pink color, and continues to spread into later stages of apple development.

Critical threshold: 100 leaf/shoot clusters are checked, which are treated when the critical threshold reaches 5% of infected leaf clusters or shoots.

 

 

Pruning should be done during the dormancy phase or in early spring, as this reduces the risk of infections. Well-ventilated locations for orchards should be chosen, tree canopies should be ventilated (not leaving dense crowns), planting varieties should be done tolerantly, fertilization should be balanced with smaller amounts of nitrogen to hinder tree growth, etc.

 

 

Special mechanical measures are mainly based on cutting and eliminating (burning) infected plant parts during winter and the growing season.

 

 

There is none

 

 

Control is based on preventive protection to avoid the disease during April and May, and later by cutting infected branches.

For protection against powdery mildew, it is recommended to use sulfur-based preparations before flowering if the temperature is above 12°C.

Fungicides based on the active substances trifloxystrobin (Zato), cyprodinil (Chorus), and difeconazole (Score) can also be used.