Apple Powdery Mildew (Podosphaera leucotricha)

Podosphaera leucotricha Caused Russeting on an Apple Fruit
Podosphaera leucotricha Caused Russeting on an Apple Fruit

The Apple Powdery Mildew, which is caused by the ascomycete  fungus Podosphaera leucotricha, represents an important disease of the tree and crop.

But despite the name, it is not only one of the most serious diseases of Apple tree, but also of Pear, Quince, and of Peach tree – from stone fruit crops.

Although it rarely causes the death of its host, its permanent presence contributes to the debilitation of the plant, its reduced strength, as well as its considerable reduction in production capacity, but also to the qualitative degradation of the produced fruits.

Therefore, the headache that causes to the grower is, more or less, expected.

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Sexual Reproduction of Podosphaera leucotricha

Podosphaera Leucotricha Sexual Ascospores produced in Cleistothecia - © A. Baudoin
Podosphaera Leucotricha Sexual Ascospores produced in Cleistothecia - © A. Baudoin
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Apple Powdery Mildew (Podosphaera leucotricha) - Classification

Apple Powdery Mildew is classified in the Kingdom of the Fungi and in the Subkingdom Dikarya. The complete taxon of the fungus is listed below.

Classification

● Domain: Eukaryota 

● Kingdom: Fungi

● Subkingdom: Dikarya 

● Phylum: Ascomycota

● Subphylum: Pezizomycotina

● Class: Leotiomycetes

● Subclass: Leotiomycetidae

● Order: Erysiphales

● Family: Erysiphaceae

● Genus: Podosphaera

● Species: P. leucotricha

➤ Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon, (1900)

● Teleomorph: Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon, (1900)

● Anamorph: Oidium farinosum Cooke, (1887)

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Apple Powdery Mildew

Apple Powdery Mildew (Podosphaera leucotricha) - Biology & Ecology

The Podosphaera leucotricha knows no borders. In this sense, it is a cosmopolitan plant pathogen, which is additionally characterized as polycyclic due to the successive secondary infections it causes through its genetic reproduction.

On the infected parts of the plant lives by absorbing its food from the host cells. As soon as mycelial mat develops enough, the fruiting bodies appear within the mycelium.

These basic biological reproduction functions, as well as its overwintering, are summarized briefly below.

Sexual Reproduction

Apple Powdery Mildew sexual propagate with the ascospores, which are produced at the end of the summer and early autumn. The spores of the fungus are contained in the bag-like asci, while the asci are carried one in the ascocarps compound fruiting bodies called also cleistothecia (or chasmothecia). Cleistothecia are presented as distinct black dots on mycelium.

Asexual Reproduction

Asexual reproduction of Podosphaera leucotricha occurs with conidia. The conidia are the asexual spores of this fungal obligate parasite, which are produced on conidiophores – specific short stalk organs. Conidia are dispersed by the wind and transported to other parts of the tree, from tree to tree and/or adjacent orchards. During the growing period, conidia reproduction is continuous up to 30 oC (86 °F), causing repeated secondary infections.

Overwintering

The overwintering of Podosphaera leucotricha is done with cleistothecia and the mycelium. However, primary infections of the fungus are not due to cleistothecia, but to mycelium. Mycelium is mainly found in the dormant buds, that infected the previous year, but sometimes also in the shoots or branches, in areas with mild winters.

Mycelium is gradually destroyed when low temperatures are at -11 oC (12.2 °F) below, while infected buds are destroyed at extremely low winter temperatures ranging from -20 to -23 oC (-4 to -9.4 °F).

However, even below -27 oC (-16.6 °F), a very small percentage of the pathogen can survive on infected plant parts and organs.

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Apple Powdery Mildew

Apple Powdery Mildew on Leaves - © Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org
Apple Powdery Mildew on Leaves - © Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Apple Powdery Mildew (Podosphaera leucotricha) - Morphology

Mycelium

Mycelium of Podosphaera leucotricha is white in color and is formed from branched mycelial hyphae, straight or curved, with a diameter of 4 to 6 μm. The mycelium appressoria (bulbous formations attached to cuticle of the host) and haustoria (the fungus ‘roots’) have a nipple shape.

Conidiophores

Conidiophores are produced at the terminal of mycelial hyphae. They are erect and straight with cylindric foot cells. The dimensions of conidiophores range in length between 41 and 185 μm and in the diameter between 9 and 12.5 μm.

Conidia

Conidia of Apple Powdery Mildew are unicellular, colorless and hyaline. They contain distinct fibrosin bodiesand are carried as a chain on conidiophores, which consists of 2 to 6 conidia. Their shape is ellipsoid to rarely slightly ovoid, with dimensions of 20 to 38 μm in length and 11 to 20 μm in diameter.

Cleistothecia

The cleistothecia are black, 75 to 100 μm in diameter. Within them are formed the asci, whose dimensions range in length between 55 to 70 μm and in the diameter between 45 to 50 μm. The asci contain 8 spores, of 20 to 36 μm in length and 12 to 15 μm in diameter each ascospore.

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Apple Powdery Mildew

Apple Powdery Mildew (Podosphaera leucotricha) - Epidemiology

Apple Powdery Mildew emerges in the spring. Primary infections begin when the infected from the previous year buds break their dormancy and the fungus resumes growth. The bud scales do not adequately protect the new vegetation and the fungus spreads to the expanding organs.

Suitable climatic conditions for the development of the pathogen are when the temperature is between 10 and 30 oC (50-86 °F) while the relative humidity is high.

For conidium germination, the relative humidity must be greater than 70%, however, this factor is not as decisive for the the progression of disease as temperatures.

In fact, the wetting of the leaves is inhibiting the conidial growth and the development of the disease. And at this point, Apple Powdery Mildew completely differs from most fungal diseases that affect the fruits and foliage of the orchad crops.

Temperatures favorable to conidium germination are those that range between 10 and 25 oC (50-77 °F) while their optimal range is between 19 and 22 oC (66-71 °F).

Under ideal conditions, the symptoms of the disease become visible to the naked eye within 48 hours.

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Apple Powdery Mildew

Apple Powdery Mildew Caused Russeting on a Pear Fruit - © N.S. Luepschen, Bugwood.org
Apple Powdery Mildew Caused Russeting on a Pear Fruit - © N.S. Luepschen, Bugwood.org

Apple Powdery Mildew (Podosphaera leucotricha) - Symptoms

The Apple Powdery Mildew infects almost all the above-ground parts and organs of susceptible plants. Although the completely destroy of them is rare, the damage it causes is often high.

The cultural practices for the Apple Powdery Mildew control include:

  • The right pruning of the trees, in a such way that allow the light to pass as far as possible inside the canopy
  • The removal by pruning in the winter of all affected shoots, as they are the basic inoculum sources for primary infections
  • The Pruning in the spring of all plant parts that are severely infected

Buds

The infected buds open later than healthy buds, 5 to 8 days, AND ARE already covered with conidia. Strong signs for their presentation is the silver-gray color of the tissues. Sometimes, when the disease spreads with particular tension, they may die-back. In some newly flower buds  that are infected by the fungus, the Podosphaera leucotricha remains in dormancy untill next spring.

Flowers

The flowers have initially silver-gray color. They have shrivel parts or even the entire flower. Petals are light green or yellowish green in color. The entire shrivel flower die-back and drops, or – if remain on tree – fail to produce fruit.

Leaves

Apple Powdery Mildew infects leaves usually from its lower surface. Initially, small, white-colored spots with unclear borders, often covered by the powder textured mycelium of the phytopathogenic fungus, appear.

Spots are gradually spreading, completely covering the lower surface and then the top.

The consequence of this is the loss of elasticity of the leaves, the presence of multi dots and their curled appearance, and even the longitudinally folded.

Under intensity conditions of disease spreading, is observed the die-back of the leaves accompanied by premature defoliation.

However, leaves are vulnerable to Apple Powdery Mildew, up to three weeks since their growing. After this time, the fungus can not infect them.

Shoots

What happens to the leaves is the same with the shoots: Podosphaera leucotricha mainly affects young and tender ones. The surface of the young shoots is covered by the powder textured mycelium of the fungus and becomes stunted. They still could have deformed appearance and usually their top is died-back.

Fruits

Apple Powdery Mildew rarely appears on the fruit. And when that happens, it’s about the immature ones. The affected fruit has a characteristic netlike russeting. That is due to the cell necrosis at the infected sites, a sample at the same time for the necrosis of the mycelium of the fungus. The fruits have a net appearance, which means that their commercial value is severe reduced.

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Apple Powdery Mildew

Apple Powdery Mildew (Podosphaera Leucotricha) - Control

Apple Powdery Mildew can be treated in a variety of ways and means, both preventive and suppressive.

Preventive Treatment

Preventive treatment refers mainly to the selection of those varieties – especially of the apple tree – which are high and natural resistant to this phytopathogenic fungus. Some of these are listed below.

Resistance Cultivars of Apple

  • Ambrosia
  • Arkansas Black
  • Arlet
  • Dayton
  • Delicious
  • Enterprise
  • Fuji
  • Gala
  • Gala Supreme
  • Grimes Golden
  • Lambourne
  • Lodi
  • Lord
  • Niagara
  • Nittany
  • Priscilla
  • Sir Prize
  • Spartan
  • Winesap
  • Yataka

Cultural Practices

The cultural practices for the Apple Powdery Mildew control include:

  • The right pruning of the trees, in a such way that allow the light to pass as far as possible inside the canopy
  • The removal by pruning in the winter of all affected shoots, as they are the basic inoculum sources for primary infections
  • The Pruning in the spring of all plant parts that are severely infected

Alternative Control

The spraying of Apple trees with emulsions of anhydrous milk fat and soybean oil has give relatively satisfactory results.

Biological Control

Indicatively, there are some active substances that can be used for the biological control of the Podosphaera leucotricha, with moderate results, however, except of Sulfur. These are:

  • Sulfur
  • Streptomyces lydicus WYEC 108
  • Bacillus amyloquefaciens strain D4747
  • Extract of Neem oil
  • Potassium bicarbonate
  • Bacillus subtilis strain QST 713
  • Bacillus pumilus strain QST 2808
  • Paraffinic oil

Chemical Control

Apple Powdery Mildew is effectively treated with the use of formulations during the winter and the spring.

Winter

  • Application of DNOC formulation (dinitroorthocresol)
  • Use of a mixture consisting of the active substances Bupirimate and Alkylphenolethoxylate

Spring

At least three interventions are performed in the spring:

  1. At the Green Tip stage
  2. At the Pink stage
  3. After Petal Fall

Some of the fungicidal active substances used to control the Apple Powdery Mildew are:

  • Benomyl
  • Carbendazim
  • Dinocap
  • Methyl Fenarimol
  • Myclobutanil
  • Thiophanate-methyl
  • Triadimefon
  • Fenarimol
  • Trifloxystrobin
  • Triflumizole

Absolutely Necessary Reminder

The reference to the active substances that can be used against Podosphaera leucotricha is for purely informative reasons. Under no circumstances is their use implied or encouraged without the approval of the local agronomist.

Contents

Apple Powdery Mildew

‘Russeting’ of Apple Fruits Caused by Podosphaera Leucotricha - © N.S. Luepschen, Bugwood.org
‘Russeting’ of Apple Fruits Caused by Podosphaera Leucotricha - © N.S. Luepschen, Bugwood.org

Apple Powdery Mildew (Podosphaera leucotricha) - Hosts

Apple Powdery Mildew infects numerous plants, both fruitful and ornamental. The most important hosts of this phytopathogenic fungi are listed below.

Fruit Trees

  • Cydonia oblonga – Quince
  • Malus domestica – Apple
  • Malus orientalis
  • Mespilus germanica – Medlar
  • Prunus persica – Peach
  • Prunus domestica – Plum
  • Pyrus calleryana – Bradford Pear
  • Pyrus communis – European Pear

Ornamental Plants

  • Malus spp. – Ornamental Species Apple
  • Photinia fraseri – Photinia
  • Photinia serratifolia – Japanese Photinia
  • Photinia serrulata – Chinese Photinia
  • Spiraea bumalda – Bumalda Spiraea
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Apple Powdery Mildew

Apple Powdery Mildew on Cluster of Leaves
Apple Powdery Mildew on Cluster of Leaves

Apple Powdery Mildew (Podosphaera leucotricha) - Geographical Distribution

It would be much easier to refer the countries where there is no Apple Powdery Mildew rather than the opposite. Indeed, it would be infinitely easier if someone remarked in advance that Podosphaera leucotricha is absent only from the Poles.

…. And the Sahara.

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Apple Powdery Mildew (Podosphaera leucotricha) - References

The References of one more (or less) profound article of ‘Kalliergeia‘, such as that for Apple Powdery Mildew, are presented by Faran Ensemble with their music composition titled Wind.

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With Pomp and Circumstance

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Οι Faran Ensemble από το Ισραήλ στέλνουν ολίγο μουσικό άνεμο: ‘Wind’

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