FORECASTING FUSARIUM HEAD SCAB OF WHEAT IN OHIO

Weather station monitoring conditions for head scab

Fusarium head scab is a serious disease of wheat when weather conditions favor the sporulation and spread of the fungal pathogen Fusarium graminearum. In 1995 and 1996 Ohio wheat producers lost $180 million in farm income to head scab due to reduced yield and poor quality grain. Management options for control of head scab are limited.

The purpose of a disease predictive system is to help wheat growers assess the risk of head scab affecting their crop during the growing season. Weather is a primary factor contributing to the development of head scab. Researchers have developed a preliminary model to assess the probability of disease being important in an area based on certain weather parameters.

Fusarium graminearum survives on corn and wheat residues and produces spores on crop residues left on the soil surface. Spores are released when crop residues retain sufficient moisture and the temperature is warm enough for spore development. Infection of the wheat head occurs at the time of flowering and requires that a sufficient number of spores land on the heads and the heads remain wet for a sufficient number of hours. Therefore by determining the weather parameters that favor sporulation of the fungus and infection of the head a disease predictive system can be assembled.

We are currently using a recently devised model that incorporates information from 50 study locations where scab severity data was obtained. The weather and disease data from these locations have been analyzed to develop a risk assessment model for scab. We are using current data from Ohio to validate this model and the data provided here is part of the validation process. The weather data is obtained from 15 weather observation stations in Ohio, Indiana and Michigan (see table below). Thus, this is a forecasting system ‘in the making’ and the information should be used for informational purposes only.

Two models are being assessed at this time.

Model 1. The first model attempts to predict the probability of head scab based on the weather that occurs prior to flowering of the crop. This model looks at the duration of precipitation in hours and the number of hours the air temperature is between 15 C and 30 C (60 F and 86 F, respectively) for 7 days prior to flowering. This is the time when the fungus is developing spores. This model has been 78% accurate in determining when the disease will not be severe.

Model 2. The second model attempts to predict the probability of scab based on the weather that occurs 7 days before and 10 days after flowering. This model looks at the number of hours when the air temperature is between 15 C and 30 C for 7 days prior to flowering and when the relative humidity is above 90% and the air temperature is between 15 C and 30 C for 10 days after flowering. This model addresses the time when the fungus is developing spores, when infection occurs and when disease develops. This model has been 84% accurate in determining when disease will be severe.

THIS DATA IS OFFERED TO THE PUBLIC FOR INFORMATIONAL PURPOSES ONLY.

DO NOT USE THIS INFORMATION TO MAKE FARM MANAGEMENT DECISIONS.

Fusarium Head Scab Risk Predictions:

The first risk prediction will be based on the weather that occurs during the seven day period prior to flowering using Model I (see first table) and the second prediction will be made about ten days later using weather information occurring during the pre- and post-flowering period using Model II. (see second table). A risk prediction will be made for early, mid and late flowering fields at each location (see tables).

Interpretation of Model Predictions:

The scab risk models predict the probability of disease occurrence; they do not predict the level of disease severity. Each model will calculate a probability of Fusarium head scab occurring at a severity level greater than 10%. This is considered to be an economic level of disease. The higher above this threshold probability level the greater the risk of head scab occurring. This is similar to a weather forecast for rain, in that the weather service predicts the occurrence of rain, not the amount of rain.

Based on known threshold probability levels the calculated probabilities will be based on the following scales:

Calculated probability Risk level

Model I 0%-35% Low

36%-57% Moderately low

58%-79% Moderately high

80%-100% High

Model II 0%-25% Low

26%-46% Moderately low

47%-67% Moderately high

68%-100% High

Posting of Fusarium Head Scab Predictions:

Head scab risk predictions will be posted as soon as weather data is available from the weather stations and the wheat crop in the area of the weather station reaches the critical flowering period. Risk predictions may change due to changes in conditions or because of additional environmental or crop development information obtained from specific locations.

2003 Fusarium Head Scab - Model I
Location	Early Flowering	Mid Flowering	Late Flowering
Jackson	Moderately high	Moderately low	High
Oxford	High	Low	Moderately high
South Charleston	Moderately low	Low	Low
Columbus	Low	Low	Low
Wooster	Low	Low	Low
Lima	Low	Low	Low
Hoytville	Low	Low	Low

2003 Fusarium Head Scab - Model II
Location	Early Flowering	Mid Flowering	Late Flowering
Jackson	Low	Low	Low
Oxford	Moderately high	Moderately low	Low
South Charleston	Low	Low	Low
Columbus	Low	Low	Low
Wooster	Low	Low	Low
Lima	Low	Low	Low
Hoytville	Low	Low	Low

Fusarium Head Scab Risk in Ohio for 2003

Head scab risk forecasting model I predicted moderately high to high risk of disease for fields in southern Ohio and for low risk of scab for other locations. The higher disease prediction in southern Ohio was due primarily to the relatively warm temperatures that occurred in early May prior to flowering. Buy mid May temperatures became cooler which reduced the risk prediction for the wheat in northern Ohio that flowered during late May

Model II predictions indicated a moderately low to low risk of scab throughout the state. It is important that users of the predictive system understand that scab risk models predict the probability of disease occurring, they do not predict the level of disease severity. Each model calculates a probability for Fusarium head scab occurrence at a severity level greater than 10%. The higher the probability level, the greater the risk of head scab occurring.

Wheat head scab survey results

A total of 148 fields were surveyed in 30 counties. Head scab can be assessed by estimating disease incidence (proportion of heads diseased in a field) or disease severity (proportion of spikelets diseased on all heads in the field) or a combination of both. Since estimations of disease incidence are more precise and easier to obtain, the survey was conducted to estimate disease incidence. The average incidence of scab per field was 8.9% with a range of 0 % to 73.3%. Over 50% of the fields surveyed had incidence levels below 5% and 76.4% had incidence levels below 10%. Only 8.1% of the fields surveyed had moderate to high levels of scab

Summary of results of the wheat head scab survey of 148 fields in 30 Ohio counties.

Incidence of head scab	Number of fields	% of fields
less than 1%	14	9.5%
1.1% to 5%	61	41.2 %
5.1% to 10	38	25.7 %
10.1% to 15%	12	8.1 %
15.1% to 20%	11	7.4 %
20.1% and up	12	8.1%

How well did the disease forecasting models predict the risk of head scab in 2003?

Of the 21 location-crop maturity risk predictions made by Model I, 6 were rated as moderately high to high. Model II had predicted only low and moderately low risk for the 21 location-crop maturity forecasts Overall, the models predicted that weather conditions were not favorable for widespread epidemic development in the northern part of the state, but disease risk was higher in the southern part of the state. The actual low level of disease (av. 8.9 % incidence) assessed in most counties corresponded relatively well with the disease risk predictions of the models. However some fields in certain counties (Henry, Licking,, Muskingum, Pickaway, Ross and Wayne Co.) had higher levels of disease than expected and in other counties (Butler, Champaign, Clinton, Shelby and Miami) levels of disease have been less than expected.

Variation in scab levels from one field to the next

Growers may experience some fields with higher levels of disease than reported in this survey due to differences in variety susceptibility, proximity of the wheat plants to a source of fungal spores in corn residue, and the total amount of time wheat heads remained wet during flowering. Additionally, relative maturity of the variety and planting date can alter the flowering date of a field causing great differences in disease levels. Thus, each field can have a different level of disease.

Relationship between scab incidence and vomitoxin levels in grain:

Vomitoxin, also known as deoxynivalenol or DON, is a toxin produced by the Fusarium fungus causing head scab on wheat. This toxin is produce by the fungus when sufficient moisture is available in infected grain and the temperatures are favorable for the growth of the fungus. Vomitoxin levels from 0 to 8 ppm have been reported in areas where scab was reported.

The scab risk forecasting models were not developed to predict vomitoxin levels in harvested grain. The time periods used by the forecasting models were those found most important for disease development. Vomitoxin can accumulate in grain during the later periods of grain filling, ripening and right up to the time of harvest. Weathering from extensive rains in mid to late June (from 10 to 17 days of rain depending on location) kept wheat heads wet and favored late season colonization of tissues in the head contributed to the overall levels of vomitoxin in the grain of some fields by harvest.