The Pathogen

Purple spot disease on asparagus spears and fern is caused by the fungus Stemphylium vesicarium. The fungus survives the winter as sexual spores (ascospores) in a sac (ascus) produced in overwintering structures (pseudothecia) that appear as small black dots on asparagus debris from the previous season. The ascospores are released from the ascus by rain and can be carried by the wind to newly emerged asparagus plants, where they cause the primary infection of the growing season. These new infections result in spores (conidia) produced by an asexual process, which in turn can cause secondary infections, a process that is repeated as long as temperatures and rainfall are favorable.

The Disease

The emergence of purple spot as a significant problem in the production of asparagus in Michigan may be due to the adoption of a no- till cultural system, whereby the dried fern from the previous season is chopped in April and left on the soil surface. This residue persists through the harvest season (mid-June) and is visible through the fern growth period (late June to September) and is the source of ascospores which start infections early in the growing season. Symptoms of the disease are: The disease appears as numerous, slightly sunken, purplish spots with brown centers occurring on harvested spears and fern. Lesions on spears are often found on the windward side, because blowing sand causes wounding which favours infection. During epidemic years spotting can occur on 60-90% of the spears and may result in rejection of the crop, especially for fresh market sales. Spots also occur on the asparagus ferns, affecting the main stem, secondary branches and needles (cladophylls). Severe infection of the fern can result in premature defoliation of the plant. Increase in the severity of purple spot disease is associated with extended periods of rainfall, fog or dew.

Text credit: Mary K. Hausbeck, Professor and Extension Specialist, Michigan State University, Department of Plant Pathology, E. Lansing

TomCast for Asparagus

Background: TOMCAST (TOMato disease foreCASTing) is a computer model based on field data that attempts to predict fungal disease development, namely Early Blight, Septoria Leaf Spot and Anthracnose on tomatoes. Field placed data loggers are recording hourly leaf wetness and temperature data. This data where analysed over a 24 hour period and may result in the formation of a Disease Severity Value (DSV); essentially an increment of disease development. As DSV accumulate, disease pressure continues to build on the crop. When the number of accumulated DSV exceed the spray interval, a fungicide application is recommended to relieve the disease pressure.

TOMCAST is derived from the original F.A.S.T. (Forecasting Alternaria solani on Tomatoes) model developed by Dr. Madden, Pennypacker, and MacNab at Pennsylvania State University (PSU). The PSU F.A.S.T. model was further modified by Dr. Pitblado at the Ridgetown College in Ontario into what we now recognize as the TOMCAST model used by Ohio State University Extension.

DSV are: A Disease Severity Value (DSV) is the unit of measure given to a specific increment of disease (early blight) development.

In other words, a DSV is a numerical representation of how fast or slow disease (early blight) is accumulating in a tomato field. The DSV is determined by two factors: leaf wetness and temperature during the “leaf wet” hours. As the number of leaf wet hours and temperature increases, DSV accumulate at a faster rate. See the Disease Severity Value Chart below.

Conversely, when there are fewer leaf wet hours and the temperature is lower, DSV accumulate slowly if at all. When the total number of accumulated DSV exceeds a preset limit, called the spray interval or threshold, a fungicide spray is recommended to protect the foliage and fruit from disease development.

The spray interval (which determines when you should spray) can range between 15-20 DSV. The exact DSV a grower should use is usually supplied by the processor and depends on the fruit quality and end use of the tomatoes. Following a 15 DSV spray interval is a conservative use of the TOMCAST system, meaning you will spray more often than a grower who uses a 19 DSV spray interval with the TOMCAST system. The tradeoff is in the number of sprays applied during the season and the potential for difference in fruit quality.

TomCast with Asparagus
Research has determined that the Tom-Cast disease forecaster is a promising alternative to calendar-based spraying of fern in commercial asparagus fields. Tom-Cast alerts growers when the environmental conditions are favorable for purple spot disease development (extended dew or rainy periods accompanied by warm temperatures). Effective fungicides applied according to the Tom-Cast disease forecaster allows growers to manage purple spot disease of asparagus, while saving money and preserving the environment.

Text credit: Jim Jasinski, TOMCAST Coordinator FOR OHIO, INDIANA, & MICHIGAN

Purple spot Infection model

The Stemphylium versicarium infection model is based on the work of SUHERI and PRICE in onions and LLorent, VILARDELL, BUGIANI, GHERARDI and MONTESINO in pear.

Infection curves for light, moderate and severe infections are computed.

With this separation into favourable, moderate favourable and very favourable situations it is to the grower to decide how big the disease pressure in a specific field will be and if he has to cover a specific infection.

Conditions:
Temperature: 11-30°C
Leaf wetness > 0 (start for infection) or rel. humidity > 90% (just if there was already an infection before calculated and is still holding)
Light infection (11-30°C), moderate infection (13-30°C), severe infection (16-30°C), an array with leaf wetness duration (see above)
Reset: if rel. humidity is lower than 70%.