Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
In this heavy disease pressure year, we are seeing issues with disease control related to fungicide choice, rates, timing, and coverage.
Fungicide choices involve evaluating cost, efficacy, and disease spectrum in relation to the type of crop and stage of growth. Additional factors include synergism between fungicides, fungicide classes and need for rotation of fungicides, maximum application allowances, and preharvest intervals. Additional decisions will be needed on rates where labels offer a range, critical timings for applications, spray volumes, and frequency of application. A complicating factor can be the loss of efficacy by fungicides or development of resistance in pathogens to specific fungicide chemistries. In each of these cases, decisions can affect the disease control obtained.
Coverage is always a concern with vegetable crops and fruits. Inadequate coverage leaves unprotected plant areas, and depending on the type of fungicide, this can increase the risk for infections and disease development.
Fungicides can be grouped into four basic mobility categories.
1) Protectant or contact fungicides are those that stay on plant surfaces. Included are many of our older fungicides (such as chlorothalonil) with many being very broad spectrum with low risk of resistance development. Contact fungicides can be used as basic disease control programs and they are added in rotation or with other classes of fungicides to improve disease control spectrums. Contact fungicides are most effective when they are evenly applied to foliage and stems. Crops where contact fungicides are used are most susceptible to disease control failures if coverage is poor. New growth will be unprotected until a new application of fungicide is made. Longevity will depend on the chemical nature of the fungicide and the susceptibility to environmental degradation, any additives used to delay degradation, additives or adjuvants that act as stickers, and rainfall.
Contact fungicides must be applied so that a large number of droplets reach the target area. Applying more water will dilute the effect of each droplet (reduced concentration) but may result in more droplets per surface area. Best protection is obtained when susceptible leaf and stem surfaces have about 500 to 600 droplets per square inch, sprayed with fine-to-medium droplets, with a concentration no less than label rate. Manufacturer’s recommendations for spray volumes and rates should be followed.
2) Locally systemic fungicides are those that are absorbed into the plant but that move very short distances such as across a leaf (often called translaminar). Examples are some of our strobilurin fungicides such as pyraclostrobin. Local systemics also require extensive coverage to be effective. Recommendations are 400 to 500 droplets per square inch, sprayed with fine-to-medium droplets, with a concentration no less than label rate. Manufacturer’s recommendations for spray volumes and rates again should be followed.
3) Xylem mobile systemic fungicides are those that move longer distances in the plant. When applied to the root system they will move to actively transpiring leaf, stem, and reproductive tissue. When applied to leaves they will move throughout the leave they were deposited on but will not move out of that leaf. If sprayed on stems they can move to nearby leaves. Spray volumes and droplet numbers are less critical with xylem mobile systemics; however, missed leaves or leaves with lower fungicide concentrations will not be protected or may have too low of rate of fungicide to be efficacious. Examples of xylem mobile systemics would be DMI fungicides such as tebuconazole.
4) Phloem mobile systemics or amphimobile systemics move in the phloem upward or downward in the plant. They have the potential to be translocated out of the leaf where they were deposited and move to other areas of the plant (in contrast, xylem mobile systems do not move out of leaves where they were deposited). Coverage is not as critical with these products, but overall concentration received by each plant is, so rates are critical. Examples of these type of systemics would be the phosphonate fungicides (examples are Phostrol, K-Phyte, ProPhyte).
It should be noted that all systemics cannot move again after translocation. That means that new growth that occurs after systemic fungicides have been translocated will not be protected.
Improving coverage for all fungicides involves paying attention to droplet size (nozzle type), nozzle number, pressure, and airflow with air assist, air blast, blower, or fan sprayers. Ground speed is also critical as well as provisions to minimize spray boom bounce. Increased rates with increased spray volume can provide more droplets thus maximizing coverage. Higher pressure or air flow can improve canopy penetration if it does not affect droplet size or drift.
In a high disease pressure year, more frequent applications will be needed to reduce the risk of new growth being infected because inoculum levels will be much higher.