Apple scab caused by the fungus Venturia inaequalis

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Contents

Introduction

History of discovery

Apple scab was first discovered in 1819 (Fries.E, 1819), but possibly existed at least two centuries prior to its discovery; as indicated by Michel Angelo Caravaggio's painting: "The Supper at Emmaus" (c. 1600) - which depicts several scab lesions on the apples in the painting, similar to apple scab symptoms.

Apple scab only appeared to be a problem once growing habits changed according to the higher demand for apples after the 1800's. Apples were eventually grown in rows and in much closer vicinity to each other. This, along with other factors such as the decreasing genetic diversity of the cultivars being used to grow apples with (resulting in more similar resistance to the fungal infection amongst plants), allowed for easier infection - since the spores have less distance to travel.

Causal organism

The causal organism is an ascomycete fungus named Venturia inaequalis (initially classified as Spilocaea pomi, and later changed to Fusicladium dendriticum), a species very closely related to Spilocaea pyracanthae (http://www.ncbi.nlm.nih.gov/pubmed/18944005). It's a heterothallic fungus that has its asexual reproduction occur once a year during the spring; and its sexual reproduction cycles during the winter. Due to this system of reproduction, Venturia inaequalis has a high genetic diversity within its populations.

Source and transfer of inoculum

The source of the inoculum are conidia, which are asexual non-motile spores of the fungus. They are haploid cells genetically identical to the haploid parent which reside on the leaves of the infected plant, and can develop into a new organism if conditions are favourable (most common in spring time), and serve in biological dispersal (Osherov N & May GS, 2001). As alluded to in the history of discovery; the transfer of the inoculum is caused by the dispersal of spores on to non-infected plants. This can be assisted by strong wind and the dispersal of leaves with spores on them, and can be further assisted by having the trees in rows and in close vicinity to each other - minimizing travel distance for the spores.

Method of infection

Venturia inaequalis will release ascospores when the temperature is warm but moist - the spring time is usually the best time for this. Once the spores are released, they are free to attach to young apple leaves through the air (assisted by wind). Once the spores have attached to the susceptible tree, they will penetrate the cuticle of the leaf and begin producing conidia. The conidia will eventually produce to the extent that it spreads throughout the leaf; this will force the spores back through the cuticle and on to the leaf where it is exposed to air once again.

Symptoms produced

The symptoms caused by Venturia inaequalis infection is seen in shoots, leaves, petioles, blossoms, sepals, and on pedicels; but is most commonly associated with the symptoms found on the fruit (i.e., apple).

When the leaves are young, the under side of the leaf can be exposed and susceptible to infection by spores, which usually happens in the spring (or in the summer if it's moist enough). Subsequently, when the leaves begin to unfold, both the upper and lower surfaces are exposed and are susceptible to infection. Lesions that aren't fully developed start as velvety brown to olive green in appearance, and have indistinct, feathery margins (these margins become more evident over time). Eventually, once the infected leaf continues to grow, the tissues surrounding the lesions will begin to thicken; causing the contour and surface of the leafs to become deformed and curled. Eventually, the lesions will become more developed and will be more clearly seen on the upper and lower sides of the leaf - this is due to the cells within the lesions dying and becoming brown in colour. Depending on the severity of the infection: anywhere from multiple lesions to the whole leaf being covered with lesions can be seen. Although, younger leaves with apple scab infection covering the surface will usually shrivel up and fall off the tree (these infected leaves can spread the infection to surrounding trees). Petiole and pedicel infections results in leaves and fruit dropping prematurely. It's also not uncommon for a secondary fungal infection to occur on top of the fungus, which gives a white colour to the lesions.

As with leaves, the symptoms of apple scabs on the fruit appear as lesions. On young fruit, the lesions appear similar to those on leaves, but once the infected fruit gets larger, they become brown and corky. If infections start earlier in the season, then the fruit can develop unevenly and become deformed or cracked - this is due to the infected areas ceasing to grow, while the uninfected continues to grow. Although the entire surface of the fruit is susceptible to becoming infected, it's usually the blossom end of the apple that becomes clustered with infection in the beginning of the season. On the other hand, apple scab infections that occur in later in the season may not be visible until the fruit are in storage. These scabs are usually smaller (they're usually rough circular black lesions ranging from 0.1 - 4 mm in diameter), because fruit that haven't developed any symptoms within weeks of harvesting are less likely to develop apple scabs after storage.

(picture of infected apple would be nice)

(picture of infected leaf would be nice)

Methods of diagnosis (especially molecular methods)

Variation in virulence among races of the pathogen and resistance in the host(s)

Physiological races of apple scab have been reported - meaning that the fungi is morphologically indistinguishable and only distinguishable by its infecting of specific species/cultivar of apple tree - but many races are observed differently depending upon the cultivated variety of apple tree used (Sierotzki & Gessler, 1998b; Koch et al., 2000) and there's little purpose or use in describing and naming the hundreds of different possible physiological races (Shay et al., 1962).

The following table provides a rough indicator as to how susceptibility of apple cultivars to apple scab:

Table 1 - Apple cultivar susceptibility to apple scab fungus (Venturia inaequalis)

(someone et al)

Apple Cultivar

Apple Scab

Resistance Ratingz

 

Apple Cultivar

Apple Scab

Resistance Ratingz

Baldwin

S

 

Monroe

S

Barry

S

 

Mutsu

HS

Beacon

S

 

Niagara

HS

Ben Davis

S

 

Northern Spy

S

Britemac

S

 

Paulared

S

Burgundy

S

 

Prima

VR

Carroll

S

 

Priscilla

VR

Cortland

HS

 

Puritan

S

Delicious

S

 

Quinte

S

Early McIntosh

S

 

Raritan

S

Empire

HS

 

Rhode Island Greening

S

Gloster

S

 

Rome Beauty

HS

Golden Delicious

S

 

Scotia

S

Granny Smith

S

 

Sir Prize

VR

Gravenstein Holly

S

 

Spartan

S

Grimes Golden

R

 

Spigold

HS

Idared

S

 

Spijon

S

Jamba

S

 

Stark Bounty

S

Jerseymac

HS

 

Stark Splendor

S

Jonagold

HS

 

Starkspur Earliblaze

S

Jonamac

S

 

Stayman

HS

Jonathan

S

 

Summerred

S

Julyred

HS

 

Twenty Ounce

S

Liberty

VR

 

Viking

S

Lodi

S

 

Wayne

S

Macoun

HS

 

Wealthy

S

McIntosh

HS

 

Wellington

S

Melrose

HS

 

Winesap

HS

Milton

S

 

Yellow Transparent

R

Mollies Delicious

S

 

York Imperial

S

zVR = very resistant. No control needed. (Very few cultivars in this category for any disease.)

MR = Moderately resistant. Control only needed with fire blight susceptible rootstocks or under high disease pressure.

R = resistant. Control only needed under high disease pressure.

S = susceptible. Control usually needed where disease is prevalent.

HS = highly susceptible. Control always needed where disease is prevalent. These cultivars should receive first priority when control is called for.

Factors that play into the aforementioned virulence/resistance of the fungus/cultivar(s) are: that the disease can grow a resistance to the fungicide(s) being used on the cultivar over time (Gao L et al, 2009); and the apple can also evolve a resistance. The Vf gene - a resistance gene in apple - is the most well known factor that contributes to the resistance of the disease in the apple (Benaouf & Parisi, 2000; Xu & Korban, 2002), but it has already been overcome in several locations (Parisi et al., 1993; Roberts & Crute, 1994), with only a single virulence factor responsible (Benaouf & Parisi, 2000).

Control methods and financial cost of disease to the relevant industry

The presence of apple scabs throughout the world causes a significant financial burden to the apple industry, and its potential for financial and methodological burden to the apple growing process can be especially serious in areas with high rainfall (especially during spring). The disease can cause premature fruit drop, reduce the quality and size of fruit, as well as cause defoliation and poor development of the fruit buds in the next season - affecting yield. Not complying with the financially burdensome procedures to prevent such symptoms can result in even greater unfavourable results, such as total crop failure in ideal weather conditions for the disease.

There are a variety of fungicide products available that will control for apple scab, some of them are protectants with little or no curative action (e.g. captan,mancozeb) to demethylation inhibitors (DMIs) such as myclobutanil or fenbuconazole with up to 4 days 'kick-back' action and some protectant action. DMI fungicides are probably the most important group of fungicides used in scab control because of their additional effective control of powdery mildew, Podosphaera leucotricha (Ell. & Ev.) Salm., which is the second most important disease of apples in the UK.

The use of fungicide has gotten to the point were researchers and industry look beyond the effectiveness of the chemical being used itself, but are also looking at how the method of spraying affects the effectiveness that the fungicide has (Krause CR et al, 2009). What is also taken into account is the fact that the disease can grow a resistance to the fungicide(s) being used over time (Gao L et al, 2009), which is why multiple fungicides with different modes of actions are used. This method not only minimizes the chances of the disease spreading, but it also minimizes the selection pressure for the disease to evolve a resistance to the fungicide(s) being used. In Victoria, Australia; a costly spraying program is needed in order to overcome the potential of apple scab infection, requiring up to 12 sprays each season.

Phytotoxicity also plays a role in choosing what type of fungicide to use, as some cultivars of apple might be allergic to specific fungicides.

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