LETTUCE DISEASE MODELS
Downy Mildew, Grey Mould and Leaf Spots of Lettuce: Lettuce is susceptible to various fungal pathogens (Bremia lactucae, Botrytis cinerea, Septoria lactucae, Cercospora longissima, Microdochium panattonianum) and bacterial pathogens (Xanthomonas campestris pv. vitians) causing damages on leafs, mould in the head and leaf spots. Because the leaf is the harvested product, Downy Mildew, Grey Mould and Leaf spots can cause considerable damage under the right environmental conditions. In the present version FieldClimate is offering Models for the Infection by Downy Mildew, an Infection Model for Anthracnose and a Risk Model for Grey Mould.
LETTUCE DOWNY MILDEW
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Downy mildew (Bremia lactucae) is a common fungal disease of lettuce in cooler growing regions. In some areas growers report that 60% (or more) of the total fungicide spray applications on a lettuce crop specifically target this disease. A program that provides good disease control, yet decreases use of downy mildew fungicides, would significantly reduce the total use of pesticides on lettuce.
Leaf symptoms of downy mildew first show as angular, pale yellow patches which are delineated by leaf veins. The first leaves of the lettuce plant or older leaves close to ground usually are the first to show symptoms. The underside of the leaf opposite the yellow patch will show white masses of spores from 7 to 14 days after infection. As the spore mass grows larger, it takes on a white, downy appearance; hence the name downy mildew.
Life Cycle and Requirements of the Organism: Infection occurs when a downy mildew spore (conidium) germinates and enters the lettuce leaf via direct penetration of epidermal cells. Entry through leaf stomata also occurs. Colonization occurs when intercellular hyphae of the fungus grow and penetrate new lettuce leaf cells, utilizing the nutrients found in these plant cells. This systemic infection can proceed rapidly. When weather conditions are right, sporulation occurs when hyphae accumulate under leaf stomata. Conidiophores bearing conidia emerge from the stomata. Wind disseminates the conidia to repeat the infection process. Conidia may also form into zoospores that either directly infect leaf tissue or become encysted for later infection.
Life Cycle of Bremia lactucae, the lettuce downy mildew pathogen Conditions for Sporulation: Like in all Downy Mildew Pathogens Sporulation takes place during night under high relative humidity and sufficient temperature.
Conditions for Spore Dispersal: Falling relative humidity or sunrise seem to be responsible for spore dispersal.
FieldClimate Downy Mildew Infection Model is based on the work done in the group of Arianne Van Bruggen in UCDavis. We adopted it to a climate where rain can be the source of leaf wetness during the summer. The key climate and epidemiologic factors are:
• Sporulation driven by Darkness
– Relative humidity (more than 85%)
– Air temperature (optimum: 15°C-23°C)
• Spore Dispersal driven by
– The upcoming morning (dew till 10:00)
– Rain (Our Experience with Downy Mildew diseases)
• Infection driven by
– Leaf Wetness
Conditions for Infection: Like other downy Mildew Pathogens leaf wetness is needed for infection. An average leaf wetness period of 4 hours was sufficient under the field conditions in central coast California to allow infections.
Central Coast downy Mildew Rule: Expect Infection if the morning dew period is lasting up to 10:00 in the morning.
FieldClimate Downy Mildew Infection Model Results
• Date and Time for
• Light Infections
• Moderate Infections
• Severe Infections
FieldClimate Downy Mildew Infection Model, Rules for Infection
• Light Sporulation and Dew results in Light Infection
• Light Infections are finished after 3 hours of leaf wetness
• Light Sporulation and 2 mm of rain can lead to Moderate Infection
• Moderate Sporulation and Dew can lead to Moderate Infection
• Moderate Sporulation and 5 mm of Rain can lead to Severe Infection
• Moderate and Severe Infections are finished after 4 hours of leaf wetness
Su, H., van Bruggen, A. H. C., and Subbarao, K. V. 2000. Spore release of Bremia lactucae on lettuce is affected by timing of light initiation and decrease in relative humidity. Phytopathology 90:67-71.
Su,H. van Bruggen, A.H.C., Subbarao, K.V. and Scherm, H. 2004. Sporulation of Bremia lactucae affected by temperature, relative humidity and wind in controlled conditions. Phytopathology 94:396-401
Padgett-Johnson, M. and Laemmlen, F. DOWNY MILDEW OF LETTUCE (Bremia lactucae): Biology, Disease Symptoms and Damage. Using the Downy Mildew Index Model for Disease Management
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Lettuce Anthracnose caused by Microdochium panattonianumis is an important disease of lettuce worldwide. The first symptoms of anthracnose appear as small circular to angular water soaked lesions on the lower leaves. Centres of lesions become necrotic and often fall out leading to a shot hole appearance. Similar sunken lesions on the veins develop and form pits along the midribs. In severe outbreak, anthracnose lesions coalesce and leaf tissues turn brown and may result in entire crop loss. The pathogen is able to survive in infested debris.
Microsclerotia have been reported as source of inoculum and can survive in the soil as long as 4 years. Rain, free water, splashing water and cool wet weather are important in dispersal of fungal propagules and conidia on the lower leaves to initiate the disease. Because the pathogen is favored by leaf wetness, cool conditions, and high humidity.
Anthracnose: Conditions for Infection: Free water from rain, dew, or sprinkler irrigation is required for spore dispersal, germination, and infection by this pathogen. The fungus infects plants through stomata or penetrates the leaves directly.
• The optimum temperature for infection is at 20° to 22°C.
• Infection can occur over a range of 5° to 30°C.
• Spores can germinate and infect in as little as 2-4 hours if there is continuous leaf wetness.
• Microsclerotia require 4-6 hours under optimum conditions for germination and infection.
• Spores are produced in lesions on leaves and are spread to other leaves primarily by splashing water.
The FieldClimate Antracnose Infection Model is initiated by rain and uses the temperature leaf wetness duration rule graphed out beside.
In FieldClimate we determine two infection ways:
a) Conidia Infection
b) Infection by microsclerotien
End of June both conidia and microsclerotien had favourable conditions to infect plant tissue.
Microdochium panattonianum B. Sutton, Galea & Price APPS Pathogen of the month February 2009
ANTHRACNOSE ON LETTUCE, Plant Diseases Washington state University extension service
LETTUCE GREY MOULD
B. cinerea infections are related to free moisture. In open field production leaf wetness is a good indicator for this. In green house production there is a problem to measure leaf wetness representative and therefore air temperature and dew point have to be used as an indicator of free moisture too.
FieldClimate is indicating the risk of Botrytis cinerea on base of leaf wetness periods and the temperature during this periods. An completed infection period is assumed after a leaf wetness period indicated by the graph beside. Such a leaf wetness period will increase the risk by 30%. If the risk is higher than 0 every leaf wetness period longer than 4 hours will increase the risk by the same relation.
A day with a leaf wetness period shorter than 4 hours is assumed to be a dry day and will reduce the risk figure by 20% of the actual value.
The same model is used for calculation of Grey mould in strawberry cultures.
The graph indicates a leaf wetness period leading to B. cinerea risk of 30%. Dry days will reduce the risk again.
Botrytis risk in Open Field Production, determined by temperature and leaf wetness periodes.
Practical use of the Grey Mould Model: The model indicates periods with risk of Botrytis infection. As longer the risk period last and as higher the risk is as higher the propability and the number of infections. The risk which is acceptable is very much depending on the marketing. Growers selling to super markets will not take any risk at the beginning of the season, knowing that infected fruits in the self will cost them their market. Wheras direct marketers might take a higher risk through all the season.