Apple Tree Diseases: Apple Scab Weather Monitoring and Forecasting

This post is in continuation to my previous post on “Apple Diseases: Apple Scab Disease Cycle”. Here we are going to discuss about the climatic conditions that help the pathogen to over winter and produce primary inoculum for the primary infection thus help us in predicting the disease in advance.

Scab weather monitoring and forecasting:

The scab fungus remained alive through winter in the dead and fallen leaves of apple on the orchard floor, and produced pseudothecia mainly in leaves which fell in September, October and November as the leaves of earlier dates did not survive in the hot autumn days and got decomposed quickly. Cool and moist climatic conditions due to rain or plenty of showers, available at higher elevation and in shady portion of the orchards were congenial for the development of pseudothecia in overwintering leaves within 30-35 days after leaf fall. When moisture was not a limiting factor (12-13 hr) and the tissues of fallen leaves had remained pliable, fungus started producing initials of pseudothecia at low temperature of 4 and 8^oC but such pseudothecia which developed slowly throughout the winter season attained full size and became mature at an optimum temperature of 15^oC. On the other hand, prolonged dry weather conditions in the winter and spring did not allow normal development of pseudothecia and a month or so delayed even the ascospore maturity.

  After the maturation of ascospores on the orchard floor, a light to moderate shower of rain is required to release the inoculum over to susceptible plant stages. This infection came from the late maturing ascospores at high elevation areas where overall development is always slow. Thus, it explains that low temperature due to snowfall is lethal to the matured ascospores and affects the infection process.

In coming post we shall be discussing about the management of disease.

           

Apple Tree Diseases: Apple Scab Disease Cycle

This post is in continuation to my previous post “Diseases of Apple: Apple Scab Description” here in this post I am discussing Apple Scab Disease Cycle.

 Disease Cycle:

The fungus Venturia inaequalis causes apple scab, which is now endemic to all apple- growing countries of the world, is particularly severe in areas having high relative humidity and rainfall.

In winter, the scab fungus over-winters in infected leaves fallen to the ground.  During autumn, the fungus begins to form tiny fruiting bodies, which are embedded in the leaves near the surface.  Sacs filled with the primary or spring spores of the fungus start to develop within the pseudothecia by late winter or early spring.  The ascospores continue to develop and mature as spring progresses.  A few spores are usually mature at the time of bud break (green tip), and maturity progresses slowly until about the tight cluster stage of blossom development.  After this time, the percentage of mature spores begins to increase rapidly whenever temperatures are favourable for tree growth.  Most ascospores have matured by the end of bloom. Mature ascospores are discharged into the air during periods of rain.  In daylight, discharge usually begins within 30 minutes after the start of the rain and is largely completed within 3 to 6 hours.  When rainfall begins at night, discharge is often delayed until daybreak, although significant night discharge can occur under some conditions.  The number of spores discharged during any one rain is determined by both the size of the potential ascospore "crop" for the season (how many leaves were infected the previous year) and the percentage of these spores that have matured since the last discharge.  Ascospore discharge usually peaks in the time from pink through bloom, and nearly all ascospores are discharged within 1 or 2 weeks after petal fall.

Ascospores are blown to nearby trees by wind currents, and they germinate in a film of water on the surface of leaves and fruit.  If surface wetness continues long enough at prevailing temperatures, growth from the germinated spore penetrates and infects the organs just beneath the outer cuticle.  Typical lesions, each bearing tens of thousands of secondary or summer spores, appear about 9 to 17 days later depending on temperature, although long periods of low humidity can delay their development.  Conidia are dispersed by splashing rain throughout the rest of the season and are capable of causing new infections.  This is due to numerous additional conidia are produced on each new lesion, repeated secondary infections have snowball or epidemic effect on disease development.

Incidence of infection is affected by the age of leaves and fruit; young leaves generally are most susceptible 1 to 5 days after unfolding and become completely resistant from the time they finish expanding until shortly before leaf drop in the autumn.  Fruit are highly susceptible until about 3 to 4 weeks after petal fall, but much longer wetting periods are required for infection to occur after this time.  Precise requirements for infection of mature fruit are not known, but limited data indicate that wetting periods must last at least 48 hours for significant infection to occur immediately before harvest.

This fungus over winters on infected leaves on the floor of the orchard. Spores are produced on these dead leaves at about the time new growth first appears in the spring. Spores are discharged during rainy periods and lodged on young leaves. If the trees are wet long enough for the spores to germinate, infection occurs. The infection originating from the over wintering stage is called primary scab. Infection depends on air temperature and continuous wetness of the plant surface. At a given temperature, the surface must be wet for a given number of hours.  The average temperatures listed in the table are calculated by averaging the hourly temperature during the wet period.  Research has shown that ascospore discharge is significantly reduced in darkness, i.e. only 5 per cent of the available ascospores will be discharged when wetness periods begin at night.  Whether this 5 percent ascospore number is significant depends upon disease pressure in particular orchard. For example if there is a large amount of over wintered inoculum, 5% of the total ascospore dose could cause severe disease problems.  However, under the disease pressure normally prevalent, the number of ascospores released at night can probably be disregarded.   When trees are dry for less than eight hours between showers, add together the periods they are wet in calculating the infection period. Spores produced on a primary scab lesion are responsible for secondary infections. Secondary infection can occur in 12 to 20 days.  Several microcomputer based disease forecasting systems incorporate the Mill's information. Apply sprays to control secondary scab if one or two primary scab lesions per tree are present.

In coming post I shall be discussing about Scab weather monitoring and forecasting

Apple Tree Diseases: Apple Scab Description

This post is a result of the comments received from Dr Kishore Khosla (Principal Scientist), my senior colleague from RHRTS, Kullu as I didn't cover this disease in blog and is one of the most important disease in the economy of apple. I am thankful to Dr Khosla for such a contribution and hope to receive corrections in future as well. His valuable suggestions will help me to write for the farmers of the state.

Though we were discussing about virus and viroid diseases in my previous post and we will be continuing on the topic again after completing series on this disease.

Description: Scab may occur on leaves, fruit, petioles, and green twigs.  Infections of the leaves and fruit are most common and obvious. Early season infections usually occur on the underside of the blossom cluster leaves because these are the first tissue surfaces to emerge from buds in the spring. Once the cluster leaves have unfolded and terminal leaves begin to develop, infections become evident on the upper surface of the leaves. Individual infections appear as rough, circular, and brown to dark olive-green spots (lesions), which often appear slightly fuzzy or velvety in texture. Lesions along the veins or margins often cause affected areas to distort or crinkle. Primary (ascospore) infections are usually limited to one or two distinct spots per leaf, whereas secondary (conidia) infections are often much more numerous. Secondary infections occasionally are so numerous that the entire surface of the leaf appears covered with scab, a condition commonly referred to as sheet scab.

Lesions on young fruits initially resemble those on leaves but turn dark brown to black and become corky or scab-like with time. Cells near lesions on young fruit may be killed, causing these regions to become deformed or cracked as they fail to grow and expand along with the remainder of the apple. Primary infections are usually limited to one or two distinct spots per fruit, often near the blossom end because it is upturned during the early stages of fruit development. Secondary infections are more frequent and numerous and the spots may be clumped or grown together, particularly if the fruit is directly beneath a concentrated source of secondary spores such as an infected leaf.  Secondary infections that occur in late summer or early fall are often numerous and relatively small in size, a symptom referred to as pinpoint scab.  Infections that occur just before harvest may be symptom-less at picking yet develop into storage scab lesions after harvest.  

In coming post I shall be discussing about the Scab Disease Cycle

Apple Tree Diseases: Apple Mosaic Virus

This is in continuation to my previous post on Diseases of apple: Management of SootyBlotch and Fly Speck. In today’s post we are going to discuss about the Apple Mosaic Virus.

Apple Mosaic

Causal Organism: Apple mosaic virus (ApMV)

Description: In the spring, pale yellow to cream-coloured areas develop on expanding leaves of infected apple trees. These areas may appear as small spots, flecks, irregular blotches, vein-net patterns, line patterns, or bands along major veins. As the season advances, they turn from pale yellow or cream to a lighter chrome yellow or white and then soon become necrotic. Severely affected leaves abscise prematurely.

Economic Impact

In general, ApMV does not have much affect on fruit production and quality. However, severely infected Lord Lambourne trees often produce fruits with cream coloured blotches.  The virus results in yield reductions varying from slight to 50 per cent.  Crop losses of up to 42 per cent were recorded in infected hazelnut and average losses of 25 per cent were recorded from almonds affected by almond mosaic, a complex disease in which ApMV plays a major etiological role.

In my coming post I shall be discussing about apple cholorotic leaf virus

Apple Tree Diseases: Sooty Blotch and Fly Speck Management

This is in continuation with my previous post on Diseases of Apple: Sooty Blotch and Fly Speck. In this post we are going to discuss about the management of the two diseases. 

These two diseases though superficial but makes a dent in the marketing of the fruit as it reduces the market value. Both the diseases can be managed easily with the scheduled spray. If a spray of Ziram @ 600g/200L water done 20 days prior to harvest the quality of the produce increases manifolds and the fruit can be sent to market with fancy grades

In coming post I shall be discussing viral and viroid diseases

Apple Tree Diseases: Sooty Blotch and Fly Speck

This is in continuation to my previous post on Diseases of apple: Management of PrematureLeaf Fall. In today’s post we are going to discuss about the Sooty Blotch and Fly Speck.

Sooty Blotch

Causal Organism: Gloeodes pomigena

Description: Sooty blotch colonies appear as shades of olive green on mature fruit. They vary from discrete almost circular colonies to large colonies with diffuse margins, which are sooty in appearance. Large amorphous colonies that cover large portions of the fruit surface result from secondary spread on the fruit.  The fungus is found on the upper surface of fruit, which can be easily removed by rubbing leaving behind slight depression. No permanent spot is found on the fruit surface. This disease is found on late maturing varieties and under storage conditions. The fungus survives from one season to the next as mycelium and pycnidia on infected twigs of apple and numerous forest trees. The fungus is dispersed as chlamydospores or conidia during rains in the spring and early summer. Most conidia are liberated by early summer, but extensive secondary spread by chlamydospores and mycelial fragments occur throughout the seasons. The infection can also be seen on new twigs, on which it remains active. During the months of May and June, low temperature and rains are necessary for disease development. For the fungus development 18.3ºC is favourable. The disease is more prevalent in low and shady area of the orchard.

Fly Speck:

Causal Organism: Zygophiala jamaicensis and Mycrotheria spp.

Description: Symptoms of flyspeck on apple are approximately described by the name. Black spots are formed on fruits. Colonies are characterized by well-defined groups of 6, 8 or sometimes 50 shiny, black, superficial pseudothecia on the surface of the fruit. This disease is found abundantly during rainy days and in less windy regions. Sooty blotch and flyspeck colonies commonly occur on the same fruit, but the colonies are mutually exclusive.

Though these diseases do not cause much harm to the fruit yet it deteriorates the market quality thus affects the price of the produce.

In my coming post I shall be discussing about Management of Sooty Blotch and Fly Speck of apple.

Apple Tree Diseases: Management of Premature Leaf Fall

This is in continuation to my previous post on Diseases of apple: Disease Monitoring System for Farmers. In today’s post we are going to discuss about the management of premature leaf fall. The holistic way of the disease management includes

1.     Phytosanitation: High and early disease incidence in shady areas is the key for the disease monitoring. The diseased leaves fallen on orchard floor must be removed from orchard floor and destroyed with the help of either by burning or decomposing in the composting pits. The cleanliness reduces the primary inoculum of the disease causing agents. This simple principle if followed the severity of the symptoms could be reduced to economic levels. Thus helps in reducing the costs.

2.     Cultural Practices: First attack of the disease takes place in shady and damp places from lower brances inner core of the tree, where the moisture remains high and air temperature is above 20^oC, sun doesn’t reach. All these conditions gives an indication that the disease causing environment essentially missing the sun, so improving the reach of the sun to the different parts of the tree will help in managing the disease. Pruning is the key. Follow proper cone shape of the tree that will enhance the sunlight penetration and air circulation thus reducing the dampness from the inner core of tree and not allowing the disease causing agent to initiate the infection without enhancing the cost to a greater level.

3.     Biological Control: Introduction of biological control agents to the phylloplane is always a good idea. Trichoderma viride and Pseudomonads help the plant in fighting with plant pathogens.

4.     Chemical Control: The epidemiological studies have now established that the disease cycle begin in first week of June in most suitable and hot spot sites. The best management strategy is through preventive method. The infection sites can be cured with few systemic fungicides. Hence the assured control measures with the tank mixing of standard brand of mancozeb (0.3%) plus carbendazim (0.05%) fungicides in first week of June to 3^rd week of June depending on the location of the orchards at different elevations. This first spray is preventive and can provide disease control for next 30 days. This spray combination can be repeated for second spray after 30 days to get the complete control over the disease. The farmers not adopting this method cannot control the disease later even with the above fungicides or number of sprays at short intervals. New generation chemicals like strobulins in combination with contact fungicide are also showing good results in the field.The apple blotch disease causing mid-season defoliation is a menace hence, needs appropriate selection of fungicides and spray at right stage for assured success in the management.

In my coming post I shall be discussing about sooty blotch of apple.