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Integration of Secondary to Primary Fungicides Program Article

Integration of Secondary Fungicides to the Primary Fungicides Program to Improve Watermelon Foliar Disease Management in Florida

Susannah Da Silva, Mathews L. Paret and Bob Hochmuth

University of Florida, North Florida Research and Education Center at Quincy and Live Oak, FL

In the recent years, watermelon production in Florida has been significantly impacted by downy mildew which is commonly referred by growers as “wildfire” due to burned appearance of large areas of foliage, powdery mildew which is increasing in incidence and severity in commercial fields over the last 6-8 years, and gummy stem blight, which continues to be problem especially when contaminated transplants, lack of appropriate and timely fungicide choices exists. Management of these foliar diseases of watermelon can be extremely challenging for watermelon producers especially in years where conducive conditions exist for one or more of these pathogens. On top of these diseases, there has been some recent occurrences of Alternaria leaf blight and Anthracnose though at a low incidence level during early-mid stages of spring season crop, which should be monitored in the coming years.

Primary fungicide program selection for watermelon is based on recent trends (historical knowledge) in disease occurrence and severity in a region and this could be different in south, central and north Florida. While a primary program could be useful, you should always expect changes between different years and this could be due to variety of factors including but not limited to quality of seeds and transplants, presence of the pathogens, and weather conditions that favor disease occurrence and spread. Being prepared to act on the finding of a pathogen with a secondary fungicide to the primary program can make a huge impact in disease management compared to only following a standard program. A summary of a comparative assessment of both situations studied in 2020 in a field trial conducted at UF-NFREC-Live Oak during spring 2020 that compared a standard program compared to a modified program that used scouting and diagnostic confirmation to add a secondary fungicide to the standard program (Table 1)

Table 1: Treatment details of the field trial conducted at UF-NFREC-Live Oak during spring 2020.

The study showed that a standard program focused on powdery mildew/gummy stem blight as key target diseases that did not take field scouting and diagnostics into perspective had very high disease severity compared to a modified program that used the primary fungicides and secondary fungicides based on early diagnostic confirmation of downy mildew in late May. The secondary fungicides used for management of downy mildew were Gavel 75 DF, Ranman and Zampro. Looking at data from the final weeks in the study, on 7/5/20 (yellow bar), four days after the final application day, the modified program had an average disease severity of 19% compared to 83% for standard program and 97% for water control. Subsequent assessments on 7/9/20 (blue bars) and 7/12/20 (green bars) also indicate the positive impact the modified program had on long-term protection of plants from foliar diseases.

Fig. 1. Progression of disease severity (%) of all foliar diseases (downy mildew, gummy stem blight, and powdery mildew) under two different spray programs and water control, from 6-24-20 to 7-1-20, and beyond the final application date from 7/5/20 to 7/12/20. The error bars represent standard error of mean.

The authors would like to note that if the primary program selected for this experiment was for downy mildew instead of powdery mildew/gummy stem blight, the outcome could have been different for the grower standard program. This is based on the fact that this trial had early downy mildew incidence and more downy mildew severity than gummy stem blight and powdery mildew. Irrespective, the study demonstrates how field scouting and diagnostics can play a significant role in improving a watermelon standard program by timely use of secondary fungicides in addition to primary fungicides.

The moral of the story is “What you expect this week may not be what you get this week”. While devising a program based on recent historical occurrences (prior years same season) of watermelon diseases is a good approach, continuous scouting of fields for early symptoms and diagnostics, and communication with local extension agents about the recent trends in finding of the diseases including in nearby counties on a routine basis are very important in appropriate secondary fungicide choices in addition to the primary fungicides program. Our experiences strongly suggest that this is a better approach than a primary program alone and evident in the study findings presented here. This is even more true in case of downy and powdery mildews where the causal pathogens could travel long distances in a short period of time, while gummy stem blight pathogen tends to have localized movement within a field.

Acknowledgement: This project is support by the Florida Watermelon Association and funded by USDA-AMS-FDACS Specialty Crops Block Grant Program. The authors would like to acknowledge Dr. Pam Roberts (UF-IFAS-SWFREC), Dr. Bill Turechek (USDA-ARS), Dr. Tatiana Sanchez (UF-IFAS-Alachua County Extension) and Mark Warren (UF-IFAS-Levy County) and other agents who routinely monitor watermelon diseases in N. Florida and have contributed to the experiment or shared critical data on disease occurrences in N. Florida)

Please contact Dr. Mathews Paret at for additional information.

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