Apple Scab Caused By Venturia Inaequalis (Cooke)

G. Winter, is among the most prevalent and economically major diseases in commercial apple orchards in regions with temperate climates worldwide. The pathogen causes lesions that have a green-olive to velvety-brown coloration when it develops on the leaves and fruits. The lesions become covered with fungal mycelium and spores which is what gives an olive-dark-green color to the spots. These spots reduce photosynthetic activity and cause young leaf drop. The development of apple scab is influenced by the temperature, the duration of the wetness of leaf, susceptibility of the plant to infection and vegetative growth stage. Apple scab may result in losses of up to 70% of the production value if left unchecked, as scabbed fruit are visually unappealing and unfit for sale. Out of about twenty cultivars of apples grown in Morocco, Golden Delicious occupies up to 50% of the surface area, followed by Starking delicious (14%) and about 5% of Gala cultivar. Management of apple scab disease is expensive due to the high number of fungicide applications (12 to 20).

In order to prevent the development of fungicide resistance in the pathogen population, a farmer needs to reduce fungicide sprays per season and alternate fungicides with different modes of action (MoA) are key strategies to prevent the development of fungicide resistance in the pathogen population. In Morocco, apple farmers have over 15 approved active ingredients for apple scab disease control (ONSSA, 2017). The three classes of systemic fungicides most widely used are methyl benzimidazole carbamates (MBC), demethylation inhibitors (DMIs) and quinone outside inhibitors (QoI). The overuse of certain fungicides has led to consumer concerns over their safety, including possible negative health effects. Fungicide application should usually start during the winter season, with copper-based contact fungicides being applied to protect against pathogen infection. If the early spring, when bud swelling and fruit set occur is particularly wet, a second preventive application should be applied.

Despite recommendations to apply fungicides early as a preventative measure, the use of fungicides later in the growing season when symptoms have already appeared has increased drastically. The MOA of each fungicide group may differ from each other depending on its chemical class. MBC fungicides (e.g. thiophanate-methyl) prevent nuclear division by blocking the polymerization of tubulin that affects mitosis (. The fungicidal activity of DMIs is due to an inhibition of fungal synthesis of cytochrome P-450- (CYP) -enzyme ergosterol (Fishel, 2005) with high levels of protective and curative efficacy against V. inaequalis. Trifloxystrobin inhibits spore germination and blocks electron transfer at the site of quinol oxidation (the Qo site) in the cytochrome bc1 enzyme complex in the respiratory chain of fungal mitochondri. A common strobilurin fungicide used in apple production is trifloxystrobin. The sensitivity of V. inaequalis to fungicides used in controlling apple scab has not been investigated in Morocco. Investigating the sensitivity of pathogens to fungicides is of great importance for crop protection. Furthermore, knowledge of sensitivity parameters will help prevent resistance of V. inaequalis to commonly used fungicide groups from occurring.