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.

Symptoms

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.

Pathogen

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

The 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

Literature:

• 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