Apple Scab Would Almost Completely Destroy Dutch Apple Orchards Without Fungicide Sprays

Apple Scab

Apple Scab

Apple scab is caused by a fungus Venturia inaqualis, which overwinters in infected leaves on the orchard floor.  Mating among different strains of the fungus occurs shortly after leaf fall and spores develop in the fallen leaves during the winter.  Spring rains cause spores to be forcibly discharged. Spores continue to mature and are discharged over a period of 5-9 weeks. If the surface of apple tissue is wet and temperatures are suitable, the spores germinate and penetrate the cuticle and outer layers of the plant, causing an infection.  The fungus grows beneath the cuticle and eventually ruptures it and forms dark green lesions.  Masses of spores are produced asexually within the lesions and become detached during rain.  Water splashes and redistributes these spores, causing secondary infections. Each leaf scab lesion is capable of producing 50,000-100,000 spores. Assuming 50,000 leaves per tree have 2% scab infection, about 50 million spores would be present on a single tree.  One spore can cause an infection. Infections early in the season can kill tissues near the fruit surface and the fruit develops unevenly as uninfected portions continue to grow.  Cracks appear in the skin and flesh and the fruit may become deformed.  Heavily infected fruit fall from the tree resulting in yield losses.  Scab lesions on harvested apples result in a lower price for growers since the commercial tolerance for scab damage approaches zero.

“Apple scab [Venturia inaequalis (CKE.) Winter] is one of the most important diseases of apple, causing considerable losses every year in many countries. Crop losses in the Netherlands caused by apple scab would be about 80% if no control measures were taken; therefore, 15-22 conventional spray applications per season are used to prevent apple yield loss under Dutch weather conditions.”

Authors: Holb, I. J., et al.
Affiliation: Department of Plant Protection, Centre of Agricultural Sciences, Debrecen University.
Title: Summer epidemics of apple scab: the relationship between measurements and their implications for the development of predictive models and threshold levels under different disease control regimes.
Source: Journal of Phytopathology. 2003. 151:335-343.

New Fungicides Take the Worry out of Controlling Mildew on Lettuce


Lettuce Downy Mildew

Downy mildew is a common fungus in most lettuce growing regions, especially during cool, moist weather.  Spores can be blown long distances.  Under favorable conditions, downy mildew is a very explosive disease, capable of appearing at high incidence in a field overnight.  When spores land on lettuce foliage, they germinate and can penetrate the lettuce leaf within three hours.  Lettuce is susceptible at all growth stages to the downy mildew pathogen.  Following penetration and establishment in the leaf, fruiting stalks grow through the leaves and branch repeatedly producing several spores on each tip, resulting in a whitish mat of millions of spores on each plant.  Affected tissues turn brown.  The fungus can penetrate to leaves internal to the wrapper leaves.  Relatively low levels of infection can downgrade a crop, cause significant trimming losses at harvest and promote decay by bacterial organisms during postharvest transport and storage.  During transit, lesions become soft and slimy as secondary decay organisms gain entrance through the tissues infected with the downy mildew fungus. High levels of disease can render a crop unmarketable.

“Incited by the obligate parasite Bremia lactucae, downy mildew is one of the most devastating diseases of lettuce worldwide. Attempts to manage this fungal disease using host-plant resistance have frequently failed due to the development of new races of the pathogen. Therefore, chemical control is of the utmost importance in humid areas where environmental conditions are very favorable for disease development.

Since the year 2000, a number of new fungicides targeting the Oomycetes, the class of fungi to which downy mildew belongs, have come to the market or are being considered for registration. It was the objective of these studies to investigate a select number of these for potential use in Florida for lettuce downy mildew control.

Of those investigated, mandipropamid and fenamidone consistently provided for high levels of control. Fluopicolide, dimethomorph, dimethomorph plus ametoctradin, cyazofamid, and propamocarb also provided significant control. With the majority of these fungicides already being labeled or close to being labeled on lettuce, it would appear that lettuce growers now have a wide array of efficacious downy mildew fungicides with differing modes of action from which to choose. This is a far cry from the situation that existed during 1989, when the EBDC fungicides were being threatened with cancellation and metalaxyl insensitivity was becoming widespread.”

Authors: Raid, R. N., and D. D. Sui.
Affiliation: University of Florida, IFAS, Everglades Research and Education Center.
Title: Management of lettuce downy mildew with fungicides.
Source: Proc. Fla. State Hort. Soc. 2012. 125:218-221.

Moldy Oranges Not Acceptable in Europe

Green mold and blue mold

Green mold and blue mold

Green mold is caused by a fungus which is ubiquitous to all citrus growing regions. Spores of this organism are airborne and large numbers are produced by the fungus on the surface of infected fruit. These spores will contaminate the packinghouse and its equipment, storage rooms, transit containers, and even the retail marketplace. Spores accumulate in water used in drenches and soak tanks. The fungus survives in the field on soil debris and produces spores that infect split and injured fruit in the tree and on the ground.

“Satsuma mandarin, one of the most economically important citrus crops in Izmir (Turkey). The most widely grown cultivar is primarily exported to European markets. Postharvest green mould caused by Penicillium digitatum and blue mould P. italicum are the most significant postharvest diseases of Satsuma mandarins. Control of the postharvest decay of mandarin is most commonly achieved by applications of synthetic fungicides.”

Authors: Yildiz, F., et al.
Affiliation: Department of Plant Protection and Horticulture, Ege University, Turkey.
Title: Effects of preharvest applications of CaCl2, 2,4-D and benomyl and postharvest hot water, yeast and fungicide treatments on development of decay on Satsuma mandarins.
Source: Phytopathology. 2005. 153:94-98.

Organic Cherry Growers Spray Fungicides More Often

Defoliated Cherry Trees

Leaf spot is the most important fungal disease of cherry.  The disease is caused by a fungus known as Coccomyces hiemalis which lives over the winter in the old leaves on the ground.  The first infection of new foliage in the early summer is caused by spores which are discharged from these old leaves.  After the fungus develops on the new leaves, more spores are produced and they may cause further spread of the disease.  Defoliation from leaf spot reduces the number of flower buds and subsequent fruit set for the following year.  Defoliated trees are less cold hardy and may be killed by low temperatures in winter.  Conventional and organic growers who spray regularly and thoroughly every year seldom suffer any serious loss from leaf spot. However, since the spray materials available for organic growers are not as effective as the synthetic chemicals used by conventional growers, the organic growers have to spray more often.

“Cherry leaf spot (CLS)… occurs worldwide and is the most prevalent disease of sour cherry in temperate zones. Epidemics caused by ascospore followed by repeated conidial cycles cause defoliation by midsummer, resulting in low fruit quality. Early defoliation delays acclimation of fruit buds and wood to cold temperatures in the fall, and reduces fruit bud survival during severe winters and fruit set the following spring.

In integrated sour cherry orchards, CLS management typically involves four to eight fungicide treatments per year, starting at petal fall and continuing until late summer. In organic orchards, only a few approved fungicides are available for CLS control, such as sulfur and copper compounds. These compounds are often less effective and more phytotoxic than synthetic fungicides used in integrated fruit growing. Therefore, in Hungarian organic sour cherry orchards, 7 to 12 sprays are applied against CLS in each season.”

Author: Holb, I. M.
Affiliation: University of Debrecen, Hungary.
Title: Effect of sanitation treatments on leaf litter density and leaf spot incidence in integrated and organic sour cherry orchards.
Source: Plant Disease. 2013. 97[7]:891-896.