A second class of fungicides used on fern are the dimethylation-inhibiting (DMI) fungicides. Examples are propiconazol and myclobutanil (Banner®, Systhane®, Folicur®). Dimethylation is a physiological process essential for the synthesis of sterols and other compounds required by the fungus to grow and reproduce. DMI fungicides inhibit a key step in this process. This is why some of these fungicides have previously been known as sterol-inhibiting fungicides (SI's). However, DMI is a more specific term that applies to the fungicides presently used on fern. DMI fungicides are locally systemic, that is, they are taken up by the young leaves to which they are applied and move systemically within them. In leatherleaf fern, DMI fungicides do not appear to move significantly from mature to immature leaves. Their systemic nature in young leaves can be very beneficial. First, it provides greater latitude in regard to the thoroughness of spray coverage required for effective disease control. Since the fungicide is distributed throughout the leaf, it better protects unfurling or expanding tissues and other areas where an effective fungicide dose may not have been deposited. Second, DMI fungicides can also suppress the growth of fungi which have already infected the leaf. In fern, this activity affects infection sites that occurred one and perhaps two to three days before the DMI fungicide was applied. Systemic activity continues for only a few days after the fungicide is applied. DMI fungicides are very useful but expensive. They should be carefully used to prevent the pathogen from developing resistance to them and to avoid phytotoxicity problems (see Ref. 6 and Sections 7.6, 7.7).

In summary, presently available fungicides do not work effectively to eradicate the anthracnose fungus or to reduce sporulation on diseased tissue. Mostly, they prevent new infections and therefore eventually reduce populations of diseased leaves. Accordingly, population densities of the anthracnose fungus are also reduced.

 

7.2 Where and When Fungicides Must Be Placed – The main purpose of fungicides is to protect fern leaves from infection. They should be applied to young leaves and coat exposed surfaces with an effective dose. Systemic fungicides must also be applied to young fern leaves to be taken up and move systemically within them. Fern leaves need to be protected throughout the critical development period from initiation through Growth Stage 6 when they are no longer susceptible to infection. This period may last up to two weeks or more. As leaves develop and expand, surfaces unprotected by fungicides are continually being exposed; these surfaces must also be protected. Once deposited on the fern leaf, some re-distribution of the fungicide takes place through leaf wetting, irrigation and rain, but this process is often inadequate to protect the new leaf surface areas exposed and the new leaves that emerge each day. Realistically, all new leaves cannot be immediately protected. However, application intervals of 3–4 days have produced acceptable results (Section 7.4). Research has shown that even when DMI (or other systemic) fungicides are used, 3–4 day application intervals should be maintained. Fungicide deposits are also eroded by rainfall or irrigation and degraded by other environmental factors. They often reach ineffective levels within about 5–7 days after application. The above considerations help explain why fern anthracnose has been very difficult to manage using traditional approaches to disease control such as occasional or periodic fungicide sprays. A partial solution to this problem is to employ shorter fungicide application intervals than those traditionally used (Section 7.4).

Protectant-type fungicides must be present on the leaf surface when spores arrive to inhibit spore germination and infection. Other fungicides are locally systemic; they are taken up by the young leaves to which they are applied and move systemically within the leaves and protect them for a short period.

The need for good spray coverage is obvious because an effective dose of fungicide is needed to protect susceptible leaf surfaces. Fungicide coverage and application rates are closely interrelated. Application methods (Section 7.3) greatly influence spray coverage. For example, it is possible to apply a correct rate of fungicide per acre without adequately covering the targeted leaves. Conversely it is also possible to cover targeted leaves with an inadequate dose of fungicide. When adequate coverage of targeted leaves is attained, the recommended rate per acre should provide an effective dose. Spray coverage and droplet patterns can be checked with water-sensitive indicating cards placed within the foliage, or by adding indicator dyes to spray tanks. Several products are available for this purpose. Your Extension Specialist can provide sources of information for these methods.

An effective dose of fungicide must be applied to the young susceptible leaves to protect them from becoming diseased. Fungicide deposits must be maintained on these leaves until they are mature and no longer susceptible to infection.

In summary, an effective concentration of fungicide must be applied to, and maintained on as many young susceptible leaves as possible to adequately suppress fungal activity and reduce new infections to acceptable levels. This can be achieved by employing more frequent fungicide applications than have traditionally been used. Application frequencies should be adjusted to reflect seasonal changes in disease control requirements (Section 10). Various chemicals (including high rates of fungicides) applied periodically to the older diseased leaves with the intent of eradicating the fungus are not effective.

Various chemicals (including high rates of fungicides) applied to the older diseased leaves with the intent of eradicating the fungus are not effective.

 

7.3 Application Methods – Effective application methods enable the correct placement of fungicides. If correctly utilized, any efficient application method should work. There are advantages and disadvantages to all and some may be better suited to one production system than another or to certain fungicide products. Considerations which cannot be compromised are correct placement on young susceptible leaves, correct concentration or application rate/acre, adequate coverage, and proper timing to protect most young fern leaves from infection. Caution: Restrictions regarding application methods permitted for specific fungicides are stated on product labels. For example, some products cannot be applied through irrigation systems.

Effective fungicide application methods provide the correct placement of fungicides at effective doses. If properly utilized, any efficient application method should work if it places effective concentrations of fungicide on most young, susceptible leaves.

 

7.3.1 Low-volume Application Methods – For fern production, we define low-volume application as obtaining adequate and uniform leaf coverage without runoff by applying fungicides in 100 to 150 gallons of water per acre, but some specialized equipment can apply fungicides in volumes down to 30 gallons per acre.  Tractor-mounted, backpack, hand-operated pressure/hose, and air-blast sprayers have all been effective for suppressing fern anthracnose. Low-volume sprayers require special efforts to obtain good coverage of the young susceptible leaves where it is needed most. For example, in a dense fern canopy, low-volume or low-pressure sprayers are unlikely to obtain good coverage of young developing leaves because sufficient amounts of spray are unlikely to penetrate the canopy. However, such equipment may be effective on small fern, new plantings, or on fern that have been recently mowed or harvested (Section 6.5). In one series of experiments, a specially-designed tractor-mounted sprayer that developed high pressure (350 lb/in²) and employed several directed drop-nozzles was found to be superior to chemigation, but equipment costs and the time needed to apply fungicides were much greater. Perhaps low-volume methods may be more appropriate for specialized, or more intense treatment of small areas, disease foci, or when spray drift needs to be carefully controlled as when spraying near workers, harvesting operations, or dwellings. Air-blast sprayers are about as effective as chemigation but subject to use when wind is not blowing. At least one electrostatic and ultra-low volume sprayers has been evaluated and found to be satisfactory.

In fern production, some fungicides are commonly applied through irrigation systems. This application method can be effective for fern anthracnose control. However, a few fungicides effective against fern anthracnose cannot presently be applied through irrigation systems. Check the product label before application

 

7.3.2 Chemigation – At first glance, chemigation for anthracnose control appears inefficient. It is amazing that it works at all. Research on row crops has shown that without special adjuvants or formulations, and using the lowest volume of water possible, only about 10% of the applied chemical becomes deposited on leaves. There seems to be little opportunity for fungicides to become more concentrated on leaves than in the water in which they are applied, so the value of 10% seems plausible. In chemigation, fungicides are often diluted to about 1/10 or more of concentrations used for low-volume sprayers. In theory, this should reduce the efficacy for control of a foliar disease. Chemigation is becoming widely used on row crops. However, in contrast to fern production, successful row crop chemigation permits moderate amounts of foliar damage by insects or disease because success equates to no significant loss in yield (peanuts, soybeans etc.) compared with low-volume application methods. Although disease control requirements for fern are more demanding, experiments have shown that chemigation (here it may also be called fungigation), properly done, can effectively suppress or control fern anthracnose. Acceptable control with fungigation depends on uniform coverage and appropriate application strategies – particularly timing and application intervals. We have now shown that that chemigation owes its effectiveness to redistribution of fungicide to young leaves, so the role of spray adjuvants is unknown and they should be used with caution.

Fungigation for anthracnose control is not the same as applying liquid fertilizer. Effective fungigation requires application of fungicide in the minimum volume of water needed to carry it, apply it to the ferns, and redistribute it within the fern canopy yet not wash off an  excessive amount of material into the soil. For effective anthracnose control, a maximum amount of fungicide should be deposited on the young foliage; it has no utility if washed to the soil. Excess water applied after the fungicide clears the system must be avoided. Excess water is detrimental and will dilute or wash off the fungicide already deposited on leaves. Indicator dyes added to fungicide concentrates and irrigation system pressure and flow meters are used to determine the exact volume of water or irrigation time needed for the fungicide to clear all zones of an irrigation system.

To be effective for anthracnose control, fungicides applied by chemigation should be applied in the smallest volume of water that provides good coverage of the young fern leaves.

Uniform coverage by the irrigation system is essential for applying pesticides and fertilizer. This requirement has been well covered in other publications (4, 5). By using catch cans as described in "Irrigation and nutrient management practices for commercial leatherleaf fern production in Florida" (4), both variation between sprinklers and system-wide irrigation uniformity can be determined. The St. Johns River Water Management District maximum water application rate for shadehouses is 0.22 inches per acre per hour (approximately 100 gallons/acre/minute). If other key factors are addressed (uniform coverage, no excess washing of foliage after the fungicide clears the system), fungigation is satisfactory for anthracnose control particularly if the fungicide can be uniformly applied within a few minutes of irrigation time (100-300 gallons/acre). Many fern growers with efficient irrigation systems have applied fungicides in about 250–300 gallons/acre with good results. Caution: some fungicides labeled for use on ferns cannot be applied by chemigation.

 

7.3.3 Special Methods for Rehabilitation – Even when fungicide programs maintain disease damage at acceptable levels, disease foci frequently remain. These foci, particularly when isolated and few in number, can be targeted for special rehabilitative treatment. Mowing, burning, marking, and avoiding contact with these areas (Sections 6 and 9) can be helpful. Populations of diseased plants within foci can be significantly reduced with supplemental fungicide treatments. It is usually cost-effective to apply supplemental treatments with backpack, tractor-mounted, or hand sprayers. Maximum frequencies and rates can be used, and, as the areas are small, little additional cost is incurred. When treating a hot spot, it is essential to treat a generous buffer zone surrounding it because disease is probably present there, but not always obvious. No rule can suffice for all situations, but an estimated guideline is to treat a buffer zone at least twice the diameter of the affected area. Follow other guidelines for fungicide application; excess rates or spray volumes are not required. The key is more frequent applications with good coverage of young leaves. It is essential to monitor treated areas to determine the effectiveness of these special efforts.

Entire ferneries or hot spots that have been severely damaged by fern anthracnose can be rehabilitated with supplemental fungicide applications.

 

7.4 Application Frequency of Fungicides – Leaf initiation and development helps determine the rate of disease increase. Unprotected, susceptible leaf tissue is constantly being exposed (Sections 4.2, 5.7 and 7.2). Moreover, fungicide deposits on treated leaves are rapidly degraded (10). Available fungicides mainly protect leaves, so there seems to be no alternative except to make fungicide application frequently enough to protect new and exposed, susceptible leaf surfaces. Experiments have shown that for much of the year, with moderate levels of disease present, 7-day application intervals will not be effective, regardless of the type of fungicide used. However, 3–4 day application intervals have produced acceptable results. For more intensive or rehabilitative programs, 3 applications per week may be justified.

The key to controlling fern anthracnose with fungicides is to use an application frequency that will protect most new leaves. The application frequency will change with the seasons of the year.

 

CAUTION - At present, there may be no provisions for such frequent applications on the label of any single fungicide labeled for leatherleaf fern; most can only be applied at 7 to 10-day intervals. Efforts are underway to modify labels of appropriate fungicides to permit shorter application intervals when needed. These uses will require better planning to meet worker re-entry requirements. Check with your Extension Agent for the latest regulations concerning this limitation.

Calendar based applications and spray programs are totally unsatisfactory for fern anthracnose management.

It is probably impractical to spray more often, but in serious disease situations, 2–3 fungicide applications per week may be justified. Such frequent applications are needed to suppress anthracnose to levels where a normal maintenance program can suffice. It should not be necessary to continue such intensive spray programs for more than a few weeks. If inspection (scouting) indicates proportions of new diseased leaves have been substantially reduced, application intervals may be lengthened accordingly (Sections 5, 7.4, 8, 9). Seasonal aspects of disease management are also important in determining application intervals. At some times of the year, depending on disease levels present, 7-day intervals can be effective. This approach requires weekly monitoring to verify that disease continues to be suppressed to acceptable levels (see Section 9).To summarize, application interval should be determined by considering several factors. Season must be considered and some type of scouting will be required to determine when and if application frequencies can be altered (Sections 9, 10). Calendar-based applications have consistently proven to be  unsatisfactory for anthracnose management.

 

7.5 Fungicide Rates – Fungicide recommendations made by University Extension Specialists reflect the product label which will state a general rate for "ornamentals" or a specific rate and application interval for leatherleaf fern. Fungicides which do not work at a rate consistent with the label, will not be recommended. Exceeding the recommended rate wastes money and places growers out of compliance with pesticide laws. Excessive application rates and frequencies have also been shown to damage fern and can have detrimental effects on vase life of harvested fronds. Label rates reflect environmental safety concerns and extensive efficacy testing. Fern anthracnose can be managed effectively by following product labels. There is no need nor justification for deviating from labeled use patterns and rates.

With most plant diseases, using less than the recommended rate, even if tank-mixed with another fungicide (when permitted by pesticide regulations) will not always provide effective protection for leaves. However, in the special case of fern anthracnose, application frequency may be somewhat more important than using maximum rates. There is evidence that minimum rates, if applied frequently enough, can be effective. This approach is presently under investigation. If effective, it will be reflected in future recommendations.

Fungicides, if improperly used, can cause serious injury to leatherleaf fern and reduce frond vase life. The type of fungicide, application rate, application frequency, and weather can also influence leaf injury caused by fungicides.

 

7.6 Fungicide Toxicity to Leatherleaf Fern - Excess fungicide rates and application frequencies can damage fern leaves. Beside chronic toxicity problems including reduced vase life (propensity for harvested fronds to wilt), excess fungicide can damage, burn, or scorch fern leaves. Some types of fungicide toxicity can occur on leaves that were immature when the fungicide was applied; others can occur on mature leaves. If fungicide label recommendations are followed, this type of injury will be relatively infrequent since the absence of phytotoxicity is an essential criterion for fungicide labeling.

Dimethylation inhibitor (DMI) fungicides have routinely caused fern injury that varies considerably with the product, in the type of injury, and the growth stages of leaves that are damaged. Injury from DMI's commonly appears one to five days after application. A bronze to yellow streaking or blotching can be produced on leaves in Growth Stage 5–6, but mature leaves can also be affected. Sometimes, DMI fungicides can damage leaves in Growth Stage 3–4 and cause a blackening of leaflet tips similar to anthracnose or some types of freezing injury. Spray adjuvants and tank-mixing with other pesticides have usually intensified phytotoxicity from DMI fungicides.

DMI fungicides (sterol-inhibiting fungicides) require special precautions when used on leatherleaf fern. Factors such as cold weather or spray adjuvants can increase the likelihood

Labels for some DMI fungicides used on leatherleaf fern suggest that trial applications be made to see if injury occurs. Some DMI's occasionally cause injury even when used at rates and frequencies consistent with the label. Others cause no injury at all except under unusual, and poorly defined, circumstances. Research is currently underway to investigate external factors that enhance injury. Cold weather is clearly one factor. Experiments have shown that fern can be injured when DMI fungicides are applied before or following intervals (1–3 days) of cold temperatures (< 4E C, 40E F); injury occurred even if subsequent temperatures remained relatively warm. Also, grower reports of injury from DMI fungicides have predominantly occurred during the winter season. For the present, it is probably best to avoid DMI fungicides during cold weather and to check periodically with your Extension Specialist regarding the current status of this problem.

Factors such as cold weather or spray adjuvants can increase the likelihood of fern injury by DMI fungicides.

DMI fungicides produce plant growth regulation effects in many crop and weed species. Growth rates and development may be seriously affected. However, none of these effects have been observed on leatherleaf fern except when unlabeled fungicides have been used routinely and at excess rates for long periods. In these instances, fungicides which caused growth and development defects in young fern fronds probably resulted from accumulation of excess amounts of these materials in the soil (6, 10). This is not unexpected. Many DMI fungicides are persistent and do not break down very rapidly, particularly in soil. Therefore, amounts toxic to fern can accumulate there. At present, it is not known how long the effects of unfavorable soil accumulations of DMI fungicides on ferns can continue.

 

7.7 Fungicides and Pathogen Resistance – Of the fungicides likely to be used for anthracnose management, only those in the class known as dimethylation-inhibitors (DMI) present significant opportunities for the anthracnose pathogen to become resistant to them. If resistance develops, the fungicide will have only minimal effects on the pathogen and the disease will not be controlled. Unfortunately, if the fungus develops resistance to one DMI fungicide, it will likely be resistant to all. Several useful DMI fungicides are currently in various stages of development and labeling for fern. It would be unfortunate if C. acutatum develops resistance to all of them. Briefly, if DMI fungicides are used exclusively, routinely, or too frequently, chances of the pathogen developing resistance to them are greatly enhanced. For this reason, DMI's should be used infrequently and always in alternation with other classes of fungicides that present negligible opportunities for pathogen resistance. Guidelines for the use of DMI fungicides in fern production have been covered in another publication (6). Pathogen resistance to other fungicides used on fern is probably not a significant possibility even with frequent and exclusive use. However, there are other reasons for alternating fungicides that are discussed in Sections 7.6 & 7.8.

Improper use patterns with any DMI fungicide may result in the anthracnose fungus becoming resistant to all DMI fungicides

 

7.8 Fungicide Programs for Managing Anthracnose – There are three major considerations for developing effective fungicide control strategies for fern anthracnose. First, and most important, is application frequency. Seven-day application intervals are not effective for suppressing or controlling anthracnose during warm weather and frequent rains; 3- or 4-day intervals are much more effective when intensive fungicide programs are required. At other times of year and in some situations, 7-day intervals will be adequate.

There are several possible strategies for using fungicides effectively without creating additional problems for leatherleaf fern production. These strategies depend upon the types of fungicides used.

Second, only effective fungicides should be used. All fungicides are not effective against fern anthracnose, but many are acceptable. Some are more effective than others, easier or more convenient to use, cover better, last longer, or cost less. However, research has shown that there is no single fungicide that will consistently provide acceptable control for fern anthracnose when applied at traditional 7- to 10-day intervals. Disease management, utilizing several fungicides effectively, is the only viable approach to anthracnose control.

Third, fungicide products should be used in an alternating sequence to take full advantage of their best properties while avoiding potential problems associated with frequent or exclusive use. Cost is also a consideration. A few examples will illustrate this approach.

All fungicides are not effective against fern anthracnose, but many are suitable when properly used. No single fungicide has been found to control anthracnose when applied at application intervals of 7 days or more.

Example 1: There are other important diseases of leatherleaf fern such as Cylindrocladium and Rhizoctonia. Not all fungicides useful for anthracnose provide acceptable control of these diseases, so it is appropriate to alternate two or three different contact-protectant fungicides that do. Cost and product preference are also important, and less costly products (if effective) can be part of such a program. Another consideration is that many fungicides contain manganese (Mn) or zinc (Zn). Although these fungicides supply some micronutrients to fern (Ref. 4 and Section 4.2.2), prolonged and continued use may result in excessive and potentially toxic concentrations of Mn and Zn in fernery soils. Thus, non-metal containing fungicides should be part of a alternating fungicide sequence. Fungicide cost is not strongly related to efficacy, but when conditions are relatively unfavorable for disease, less costly fungicides can be included in such a program and still provide acceptable results. In this example, two or three contact-type, protectant fungicides might be alternately applied at appropriate frequencies Sections (5.8, 7.4, 10.3). There is no compelling reason to use one fungicide for one application and different one for the next, or the same one twice in a row. Alternating fungicides is the key. Products may be alternated as well, and results should be similar.

Example 2: In this example, a DMI fungicide is introduced into the program. This approach is very different from Example 1. It requires more planning and scheduling. For reasons presented in Sections 7.2 to 7.7, DMI's not only tend to be more effective but also more expensive and complicated to use. The DMI will make a difference because its systemic properties make it more effective in preventing infections during the period in which that particular application remains active (Sections 7.2 - 7.3). For reasons outlined in Ref. 6 and Section 7.6 – 7.7, DMI fungicides should not be used often or routinely, so alternating them with other fungicides is essential. Alternating one DMI fungicide with another is not recommended for the same reasons. Occasional or periodic use of a DMI fungicide in alternation with contact protectant fungicides (perhaps 1 week out of 4) can be advantageous particularly under conditions favorable for disease development. Some DMI product labels specify how often they can be applied per crop or per year, but some do not. We do not recommend frequent or prolonged use of DMI fungicides to leatherleaf fern (6). Finally, DMI fungicides should be used with caution or not at all when cold weather is possible (Section 7.6).

DMI fungicides, in particular, present special and potentially serious problems if improperly used

A typical program that employs a DMI fungicide might be as follows: Two applications per week are made. The first week, a DMI fungicide is applied; later in the week, an ethylene-bis-dithio-carbamate (EBDC) type fungicide (example: mancozeb) is also applied. The second week, two applications are also made; one might be a chlorothalonil product and the second, an EBDC-type fungicide. The third week is the same as the second week. For the fourth week, the cycle begins again with the DMI fungicide followed later in the week by an EBDC fungicide. This approach exploits the advantages of DMI fungicides and minimizes their potential hazards.

These fungicide programs are more complex than those traditionally used and require careful planning to avoid conflicts with harvesting and worker protection and re-entry standards.

The fungicide program examples described above are more complex than those traditionally used. They require more careful planning to avoid conflicts with harvesting activities and worker protection and re-entry standards. It is beyond the scope of this manual to list specific fungicides (except to provide examples) or make fungicide recommendations because product availability and labels will change. New fungicides will become available. Some tank-mixes have appeared particularly promising, but more research is needed in this area. Problems encountered after long-term use or when mixing fungicides may also be identified. For example, unsightly fungicide residues on harvested ferns that may be difficult to remove is an important consideration that may require special planning. Your Extension  Specialist or fungicide supplier can assist you regarding the current status of products, and fungicide programs. Extension publications, newsletters, and current product information are key sources. With the information gained from this manual, you should be able to effectively conduct your own evaluation of products and management techniques and avoid most potential problems associated with fungicide use.

This manual does not recommend specific fungicides for fern anthracnose control. Contact your Extension Specialist or fungicide supplier for information on the current status of products and recommended fungicide programs for leatherleaf fern and fern anthracnose.