Some Mistakes in Fungicide Application to Manage Rice Diseases

by Yeshi Wamishe, Extension Rice Pathologist - July 24, 2020

It is tough to go back in time and correct the mistakes we’ve already made. However, our mistakes should be learning opportunities. Below are some mistakes one may make with rice related diseases and fungicides.

Planting large areas without adequate water resource

If you have already planted a large area beyond the capacity of your water resource or pump capacity, your crop may suffer a water shortage and become more intolerant to diseases. Blast is a disease of importance in such situations. To manage blast effectively, application of fungicides alone may not give the desired protection if the water depth in the field is shallow as the chemical alone may sometimes be overwhelmed by the disease pressure.  

Failure to have soil nutrient level tested and get corrected ahead of the season

Different soil types and fields may have diverse levels of macro-and micro-nutrients. The essential nutrients are requisite for the normal physiological activities and growth of rice plants. Rice can show disease-like symptoms due to the deficiencies in nitrogen, sulfur, zinc, phosphorus, and potassium among others. Good examples are --brown spot and stem rot diseases that can be corrected with addition of nutrients such as potassium.

Failure to have a field history

Knowing the field history with previous diseases saves you money. For example- if sheath blight appears at early boot and is already at threshold for treatment, you may plan for fungicides. However, if you know your field also has a history of kernel smut or false smut, you can reduce the application frequency from two to one by pairing fungicides targeting more than one disease.  

Failure to know the susceptibility/ resistance level of your variety

In your variety selection, choosing varieties with lower susceptibility to a known disease in a field with a history is important. For example, planting blast susceptible or very susceptible rice variety in a field with a history or to fields prone to blast may require more attention to your management including application of protective fungicides. Failure to match variety with fields can increase your input cost. 

Waiting too long to scout

Leaf blast and sheath blight require scouting. Scouting for leaf blast in fields planted with susceptible rice should start from late tilling. Scouting for sheath blight often starts from early reproductive stages. Scouting for brown spot or stem rot helps for timely correction of potassium deficiency. Although weather forecast may sometimes be off, weather forecast can help us with what to expect related to disease situations.

Incorrect disease diagnosis

Any problem that causes abnormal physiological activity may be referred as disease. However, all field problems are not caused by microbes such as pathogenic fungi, bacteria, etc. The rice crop can get sick from unfavorable weather conditions, herbicide damage, insect damage, nutrient deficiencies or our mismanagement. Wrong diagnosis and applying fungicides for the wrong cause incurs unnecessary cost.

Applying fungicides in poorly managed fields

If your field is poorly managed and you foresee the narrow profit or no profit, it will not be wise to add additional input cost related to fungicide application. Fungicides work better and benefit better in well managed fields.  

Failure to read fungicides labels

Read product labels. Labels are the rule! Particularly when mixing a herbicide or an insecticide with a fungicide, pre-check their compatibility.  Applications of chemicals at flowering are highly discouraged.  

Waiting too long to plan for fungicides

Timely fungicide application is key. We should not start planning for a fungicide when it is time to apply.         

Waiting too long for application (missing the correct timing)

In fungicide application timing is key to manage rice diseases. Applying fungicides too early or too late has disadvantages.  It has to be timed right. For example, if you apply protective fungicide for the smuts too early it may not last long enough to serve the purpose; if applied late, you may not get the desired protection because the damage may have been already done. Same for neck/panicle/collar blast. Moreover, if you go too early for sheath blight, it may not last long enough to protect the upper three leaves in warm and wet seasons as the crop gets near maturity.  

Failure to use adequate volume of water to deliver fungicides

The crop canopy needs to have adequate fungicide coverage to get the desired disease suppression. Some think morning dew to serve in re-distribution of the fungicides; some think crop oils can do the job. As far as our research tells, nothing replaces good canopy coverage.  For areal application 5-7GPA of water and for ground application up to 10 GPA are recommended.   

Failure to use the correct rates and fungicide application frequency

Generally, in pathology, the principle is to stop or eradicate the pathogen if possible. As a result, higher labeled rates of fungicides are preferred. In sheath blight suppression, research has shown Azoxystrobin at 12.5 oz rate suppressing the disease for 28 plus days, 10 oz for 21 days, and 8 oz for 15 days. However, we are not sure if the lower rates encourage development of insensitive strains of the pathogen which may lead to fungicide resistance.  

Fear-based application and application for yield increase

Development of fungicide resistance should always be a concern. Since there are limited fungicides with limited modes of action (FRAC code 3, 7, 11), try to avoid automatic fungicide application as a matter of habit.

Failure to practice best cultural practices

Overall, well managed fields often have less problems. Rice fields that started with clean seeds, early planting (when possible), optimum seeding, nitrogen and potassium rates, and clean fields from weeds with a good flood depth are often healthier.