Do you ever feel like you’ve done all you could and accomplished very little? I am sure you can relate to that when it comes to Fusarium control in garden mums.
Fusarium spores are very resistant to cold and drying, thus easily overwinter in flower beds and on plant debris. Most plant pathologists will tell you that sterilizing beds and eliminating all Fusarium spores is very hard and practically impossible. This is not very encouraging for growers who experienced Fusarium problems this year and are facing the possibility of infecting plants again next season. We had ideal conditions for Fusarium development this year, record breaking temperatures, high humidity and excessive moisture in some parts of the country. As a result, we had the worst Fusarium wilt outbreak on garden mums in recent years. Whether we like it or not we should assume that Fusarium will come back next year and we must develop a plan to control it. First, we need to make sure that we have correct diagnosis. Often Fusarium symptoms on garden mums could be mistaken for Pythium and that could lead to applying wrong fungicides. It is always easiest to diagnose Fusarium in early stages. Usually a single branch will turn yellow and wilt rather than the entire plant all at once like with Pythium aphanidermatum.
Another main difference is that in case of Fusarium the root system remains healthy, while there is a reddish-brown discoloration inside the stem in the xylem. For Pythium, the root system turns brown and softens but there is no discoloration inside the stem. If you still not sure which disease is decimating your crop you might resort to a lab diagnosis which is always more accurate.
What steps do we take to minimize Fusarium impact on our crops for next season?
We should start with cultural changes that could improve not only the garden mums but other crops as well:
- Always use brand new trays in propagation, recycle the old trays with Fusarium
- Clean and maintain weed free flower beds, Fusarium spores will survive on plant debris and weeds growing around greenhouses.
- Maintain pH of growing media above 6.2, some research suggests that maintaining higher pH will prevent Fusarium but be aware of potential Iron deficiency if pH is too high.
- Avoid using ammonium based fertilizers that will make plants softer and lower pH of the growing media
- Don’t allow plants to sit in water, fix drainage issues before spacing garden mums, the wet areas tend to harbor more Fusarium inoculum.
- Use clean water supply, you could infect your entire crop with run-off irrigation water that wasn’t treated.
- Don’t allow plants to root into the ground beds that previously had plants infected by Fusarium
What if the cultural methods fail to make the difference and you must reach for other means to control Fusarium?
Not surprisingly I didn’t get many returned e-mails or phone calls from chemical companies when I asked about their best fungicides for Fusarium control. Apparently, the options are very limited. After researching different publications and disease control guides the following fungicides were the highest rated for Fusarium control:
- Medallion (Phenylpyrrole)
- Heritage (Strobilurins)
- Cleary 3336, OHP 6672 (Thiophanate-methyl)
- Empress Intrinsic (Pyraclostrobin)
- Orkestra Intrinsic (Pyraclostrobin)
- Chipco 26019 (Dicarboximides)
- Pegeant Intrinsic (Pyridine + Strobilurin)
- Terraguard (Imidazole)
- Mural (Azoxystrobin + Benzovindiflupyr)
The new Orkestra and Empress Intrinsic from BASF look very promising but there is not much data available for garden mums and we are still waiting for registration in some of the states. Syngenta’s Mural is also labeled for Fusarium but it remains to be seen how effective it is for Fusarium control in chrysanthemums.
We should also consider biological control which seems to be gaining popularity and rightly so. The main biological agent that is widely used for Fusarium control is Trichoderma fungi. It is sold under the brand names of Root Shield and Asparello T34.
Margery Daughtrey from Cornell University was kind to share results of the comparative trial with Trichoderma fungi and Heritage WP for Fusarium Chrysanthemi control in garden mums. There were six different treatments. The growing medium in all pots (except for the non-inoculated control) received inoculum of Fusarium to each dibbled planting hole prior to inserting the rooted cutting.
Treatment 1. Non-inoculated, non-treated control
Treatment 2. Inoculated, non-treated control
Treatment 3. ASPERELLO T34 (Trichoderma asparellum) Biocontrol incorporated in growing medium and drenched preplant; cuttings also dipped at planting and drenched one week after planting.
Treatment 4. ASPERELLO T34 Biocontrol incorporated in growing medium only
Treatment 5. Heritage WP applied only once, as a drench at planting
Treatment 6. RootShield (Trichoderma harzianum) Granules incorporated in growing medium.
Chrysanthemum plants were rated for top quality using a 1 to 5 scale in which 5=good size, good color, 4=one leaf with chlorosis, 3=one wilted branch, 2=more than 1 branch wilted (dying) and 1=dead plant.
The results of biocontrol with Asparello T34 and RootShield, both incorporated in growing medium shortly before transplanting (treatments 4 and 5) were very impressive comparing to single Heritage WP drench at transplant. The Trichoderma fungi seems to control Fusarium Chrysanthemi in the growing medium for the duration of the crop, where Heritage WP provided protection only for a short time after application.
As one could easily conclude from the above trial, the biological control with RootShield would provide long lasting control and fungicides like Heritage are short lasting and need to be reapplied. So, what worked for some of the growers that were successful in dealing with Fusarium? The key is prevention:
- Preventative fungicide drenches or sprenches at least every 3 weeks
- Suggested fungicide schedule for 8” crop with 11 week grow time: week 2, week 5, week 8, week 10
- Pretreating flower beds with fungicides before spacing garden mums
- Incorporating RootShield into growing medium
- Maintaining clean beds with good drainage
These fungicide drenches scheduled toward the end of the growing cycle are a must. Most Fusarium cases are always reported before cracking color when plants become more susceptible.