Grapevine powdery mildew is a pathogenic fungal disease of grapes, the management of which is a yearly challenge for California grape growers. Since the mid-80’s, multiple fungicides have been registered for grapevine powdery mildew management. The first of these was Bayleton, a demethylation inhibitor (DMI). It controlled mildew well for two seasons, after which growers noticed a sharp decrease in efficacy. Growers were experiencing powdery mildew fungicide resistance firsthand. Resistance to Bayleton prompted a survey which showed that mildew developed resistance to fungicides in many California vineyards. Since then, we have learned a lot about powdery mildew biology, fungicides, and resistance management.
What is fungicide resistance?
Mildew is said to be resistant to a fungicide after exposure to the fungicide fails to control the organism. Resistance implies that the mildew has developed the ability to detoxify the fungicide by changing either the intermediates within a metabolic pathway or by changing protein binding sites.
Currently grape growers (both conventional and organic) have a diverse chemical arsenal to control mildew, including several compounds with different modes of action. Compounds having a similar mode of action are said to be in the same class, which is to say that they “attack” or disrupt the same metabolic processes to affect the growth of the fungus. Repeated use of fungicides from the same class—one application after another—within a season and over seasons promotes the rapid development of resistant mildew populations. The development of resistant mildew forces growers to rely on the remaining fungicide classes that remain effective, which could lead to overuse of those fungicides, and the loss of another class due to resistance. The loss of any fungicide class also limits a grower’s ability to tailor control programs to their production systems, including cultivar, trellis type and end product. Thus, yield and quality can be compromised by the emergence of fungicide resistant mildew populations, especially on the most susceptible cultivars, such as Fiesta, Carignane, and Chardonnay. Thus, it is important to understand the different modes of action between classes to develop an effective mildew control program that minimizes the risk of resistance.
Fungicides and modes of action
There are currently > 20 products registered for powdery mildew control, and six different fungicide classes. Some, like sulfur, have been used for centuries to manage plant diseases while others are newly registered and still being field tested. Only those that are currently registered in California for commercial grape production will be discussed.
The different modes of action can be relatively specific, interfering with a biological pathway at numerous sites, for example, or rather crude as with contact oils which simply stifle fungal growth. The alternate use of products with different modes of action will help minimize powdery mildew resistance.
· Sulfur: Elemental sulfur (dusting or wettable) has been used to manage grapevine powdery mildew for almost 200 years. Sulfur’s exact mode of action on powdery mildew growth is not known but it is believed to act as a multi-site inhibitor either through contact, inhibiting spore germination or through toxic vapors that affect respiration. Normally applied every 7-10 days, sulfur is a foliar protectant during environmental conditions conducive to powdery mildew growth. It should be noted that foliage burn (phytotoxicity) might occur when temperatures fluctuate between lows and highs in short periods of time (from day to day), not allowing for foliage to acclimate.
· Demethylation Inhibitors (DMI’s): Commercially available since 1982, Bayleton was the first DMI registered for controlling grape powdery mildew. Since then, several other DMI’s have been developed and registered, becoming one of the most important fungicide groups for all agriculture commodities. Most DMI’s are single site inhibitors of ergosterol biosynthesis, a compound needed for normal fungal cell membrane function. Their unique selectivity also makes them vulnerable to resistance within powdery mildew populations. Synonyms include SBI’s (sterol biosynthesis inhibitors) and SI’s (sterol inhibitors). Their interval lengths range from 14 to 21 days depending on chemistry and are locally systemic.
· Strobilurins: These are secondary metabolites derived from wood rotting fungi such as Strobilurus tenacellus. Fungi produce these compounds for their own benefit to inhibit the growth of other fungi or bacteria competing for nutrients in natural settings. Strobilurins work by disrupting the electron transport chain within the mitochondria of the target organism. Mitochondria are responsible for producing the energy requirement needed for spore germination and fungal growth. In addition to controlling powdery mildew, these fungicides also control Phomopsis, and downey mildew. Their interval lengths for these products range from 14 to 21 days.
· Quinolines: Quintec is the only member of this class. It is a unique product that is capable of being redistributed over foliage and fruit, allowing for protection on tissue not directly contacted by the product. It is thought that a vapor forms and moves over the plant surface and gets reabsorbed by tissue adjacent to that initially sprayed. It acts as a cell signaling disrupter early in the infection stage. It should not be applied more than two consecutive times and has a 21 day interval.
· Contact Products: There are two groups that terminate powdery mildew growth by contact, the oils and the bicarbonates. JMS stylet oil and Saf-T-Side are oils used on 14-18 day interval between applications. Oils should not be used in conjunction with sulfur or following a sulfur application unless at least ten days have elapsed since the last application. The mode of action for oils is strictly one of contact with the fungus and disruption of the cell wall
Bicarbonates such as Kaligreen are can be mixed with sulfur in order to eradicate populations of mildew. These products are used on a seven to ten day schedule. Bicarbonates work as cell wall disrupters upon contact. Adequate water should be used for good coverage. Bicarbonates are used on a 7 day interval.
· Biologicals: Biologicals are products that occur naturally and may be available to organic programs. Serenade, derived from Bacillus subtilus is a fermentation product produced by the bacterium to inhibit growth of competing organisms. Its mode of action is such that it inhibits spore germination and fungal growth by releasing proteins as it grows.
· Systemic Acquired Resistance (SAR): Phytoalexins are toxic compounds that are produced by healthy plant cells and released near the site of injury or invasion by fungal pathogens. Injury of plant cells induces this natural plant response so it can protect other areas that have not yet been attacked. The result is a systemic acquired resistance brought about by an injury at a single site. Chemical products have been developed that also bring about this host defense and aid in protecting uninjured areas. Once the signal is triggered, the plant readies itself for infection by producing thicker cell walls, more cuticle layers, releasing antifungal enzymes, or causing surrounding cells near the infected cell to die.
Resistance to sulfurhas never been found in any powdery mildew fungi, probably due to the multi-site mode of action. However, resistance to fungicides with a single mode of action, such as the DMI’s and Strobilurins, can be expected if they are used exclusively to control powdery mildew. Once resistance occurs the potential for it to remain in the population is greater and different modes of action are needed to “knockout” resistant populations.
Key to resistance management is the rotation of chemicals within a growing season that have different modes of action. For example, using DMI’s and Strobilurins in combination with sulfur will control powdery mildew and decrease the chance of resistance to either class of chemistries. However, the use of different brands of DMI’s followed one after the other will result in powdery mildew resistance since the all target the same metabolic site. Similarly, Strobilurins have the same mode of action as each other and used exclusively in the same season will and result in resistance.
A good resistance management program for a Thompson Seedless grower would be as follows: Four to five applications of sulfur followed by either two applications of a DMI or Storbilurin starting at bloom and then finishing the season with sulfur or combinations of other classes of fungicides.
Good spray coverage is critically important, regardless of what chemical is used. For susceptible cultivars like Carignane that develop dense canopies, it is important to choose the right product throughout the year to obtain excellent season-long control. A longer lasting product might be a better choice in this situation than sulfur when coverage is an issue. Proper timing, amounts and alternating chemicals throughout the growing season is the key to controlling powdery mildew and preventing resistance.
Powdery mildew on young grape berries.