Apple Phytophthora Rot: Symptom Identification, Control and Fungicide Options

Apple Phytophthora rot, also called collar rot and fruit rot, is one of the important diseases of apple, and hot, humid weather easily triggers it. So what symptoms does apple Phytophthora rot have? What control methods are there? What fungicides are used?

Apple Phytophthora rot is one of the few apple tree diseases caused by lower fungi, and it differs from common diseases such as rust, powdery mildew, brown spot, early defoliation and scab, and its control techniques (especially the choice of fungicides) differ considerably. Today I will explain it to you.

I. Damage symptoms

Phytophthora rot mainly damages fruit, the root collar and leaves. After the fruit is infected, irregular dark red lesions of uneven depth appear on the fruit surface, with unclear edges that look water-soaked. Sometimes the lesion separates from the flesh, and the surface appears white and waxy. After the flesh turns brown and rots, the fruit shape does not change and it becomes ball-like and elastic. Diseased fruit drops very easily, and finally loses water and shrivels into a mummified fruit. At cracks or wounds on diseased fruit, white cottony mycelium can be seen. On fruit of the Delicious (Yuan Shuai) strain of apple, lesions generally appear first at the five-lobed (calyx) area, with brown dry spots appearing between two lobes, surrounded by a red halo, then spreading to two or three lobes and gradually to the whole fruit. In severe cases the fruit drops off and loses its food value. When seedlings and the root collar of large trees are damaged, the cortex rots brown; after the lesion girdles the trunk, the branches above ground sprout slowly, the leaves are small and yellow, and finally the whole plant wilts and the branches and trunk die. When leaves are damaged, irregular grayish brown or dark brown water-soaked lesions appear, mostly starting from the leaf margin or middle, and in humid conditions the lesions expand rapidly and cause the whole leaf to rot.

II. Disease development pattern

1. Timing:

Fruit can be infected throughout the whole growing period, and after each rainfall there is one infection and disease peak.

2. Variety:

Red Fuji is commonly affected, and dwarfing rootstocks are more prone to collar rot than standard rootstocks.

3. Climate:

Under high temperature and high humidity, the disease develops rapidly and spreads quickly.

4. Cultivation factors:

Disease is more severe in orchards with a dense canopy, overgrown weeds, weak tree vigor, low-lying terrain and wounds on the root collar of apple trees, especially in orchards heavily irrigated with large amounts of water before rain.

III. Control methods

1. Choose and plant resistant varieties. Red Star (Starking) and Golden Delicious are susceptible, while Fuji, Ralls Janet (Guoguang) and Jonagold are relatively resistant.

2. Strengthen cultivation management, thin fruit in time, remove diseased fruit and diseased leaves, and bury them deeply or burn them in a centralized way. Remove overly dense branches and drooping branches in time to improve ventilation and light penetration. Covering the ground under the canopy with plastic film or grass can prevent pathogens in the soil from splashing onto the fruit; orchards with severe disease should not be intercropped with vegetables, to reduce conditions favorable for the disease.

3. Clean up fallen fruit in time and remove diseased fruit and diseased leaves from the trees for centralized treatment; when plowing and weeding, be careful not to bruise the root collar.

4. The sites infected by Phytophthora rot are mainly the fruit on main branches below one meter, especially fruit close to the ground, which is easily infected. Therefore when bending branches, especially the lower branches, orchards with Phytophthora rot must be careful not to bend the branches at too wide an angle. Where the disease has already been found, promptly lift the main branches up by more than 60 cm, so that the Phytophthora rot pathogen has no chance to infect.

Reference control fungicides:

47% dimethomorph · zoxamide? suspension concentrate (low toxicity) use 50-60 ml/mu spray

40% dimethomorph · fluazinam suspension concentrate (low toxicity) use 33-40 ml/mu spray

687.5 g/L fluopicolide · propamocarb suspension concentrate (low toxicity) use 60-75 ml/mu spray

46% copper hydroxide water-dispersible granules (low toxicity) use 25-30 g/mu spray

52.5% oxathiapiprolin? · cymoxanil water-dispersible granules (low toxicity) use 20-40 g/mu spray

72% cymoxanil · mancozeb wettable powder (low toxicity) use 133-180 g/mu spray

100 billion spores/g Bacillus subtilis wettable powder (low toxicity) use 10-14 g/mu spray