Rice Fungicide Seed Treatment: To Apply or Not Apply

by Yeshi Wamishe, Extension Rice Plant Pathologist and Jarrod Hardke, Rice Extension Agronomist - April 12, 2021

It is often important to ask why and when it is required to use a fungicide seed treatment for rice.  Your field history is a big factor in your decision.  If your field had a history of seedling death caused by water molds such as Pythium spp (Figure 1), or seedling blight caused by Rhizoctonia spp., it is worth it to use fungicides. 

Sometimes you may not be sure if your field had a history of seedling diseases.  Start observing carefully.  If the seedling density is not as expected and you see mold-looking structures around dead seedlings or blight at the soil line, you need to be suspicious of seedling diseases.  Seedling disease identification is complex during the early rice season when the temperatures are often lower than needed for timely seedling emergence while at the same time stressed by herbicide effect or insect damage.  

For several years to date, we have been evaluating different fungicide seed treatments with artificial inoculation at the Rice Research and Extension Center (RREC) in Stuttgart using different industrial products.  For the artificial inoculation, we used mostly Rhizoctonia solani AG-11and recently R. solani AG-9 which is more aggressive than AG-11.  Our planting dates were consistently between the 2nd to early 3rd week of April.  Often, it took nearly two weeks before more than ~50% seedling emergence.  

Most of our tests have shown the importance of seed treatment fungicides in the presence of the pathogen inoculated. There were statistically significant differences among the fungicide-treated and untreated plots regardless of differences among the products.  Please refer to rice seedling diseases in MP154  for previous and newer products.  Vibrance RST is a recent product that contains four different modes of action (mefenoxam, fludioxonil, azoxystrobin, and sedaxane).  In our test, it had shown significantly higher seedling count compared to mefenoxam + fludioxonil (Apron XL and Maxim 4 FS) and metalaxyl + fludioxonil (Allegiance FL and Maxim 4 FS).  Recently a new seed treatment fungicide Zeltera (not included in MP154, 2020) has been labeled for rice.  It also has shown excellent protection to rice from seedling diseases specifically to Rhizoctonia.  Both of these newer products were tested using Rhizoctonia solani AG-11 and AG-9 separately or in combination.   

Generally, seed treatment fungicides are relatively cheap and important tools for managing rice seedling diseases caused by particularly Rhizoctonia seedling blight.  However, there is no proven evidence so far, on our part that showed seedling protection lasts indefinitely.  Up to three weeks of protection is estimated from seed treatment fungicides. 

If we do plant under favorable temperatures and adequate soil moisture for faster seedling emergence, we may not need to use fungicides for seed treatment. Note that cold and wet environmental conditions are favorable for seedling diseases.  In some cases, seedlings die from seed-borne or soilborne fungi and molds even after emergence under warm and humid conditions depending on the inoculum density and soil type.  Fields cultivated with rice after rice or in relatively heavy rice residue or under minimum or reduced tillage can be prone to seedling disease complex.  Also, water-seeded fields can have more seedling diseases than dry-seeded fields.  However, seed treatment fungicides are not recommended for seeds to be used for water seeding.  Planting depth also plays some role in the severity of seedling diseases.  In the past, higher rates of seed treatment containing mefenoxam, fludioxonil, metalaxyl, trifloxystrobin in a combination of one or two were used for early planting in fields with a history of severe disease situations.  Remember salty fields may have seedling stand problems as well. Seed treatment fungicides do not solve salt problems. The federal label requires that fungicide seed treatment be applied only using commercial seed treatment equipment. Read product labels for safety guides. Labels are the rule. 

Fig 1. Disease affecting rice seedling caused by Pythium spp.