Vinews

No. 5 — March 22, 2021

Contents:

• Cross-resistance to Insecticides Develops in Fruit Flies Drosophila spp. in a Missouri Vineyard
• Grape Leaf Rust (Phakopsora euvitis)

Cross-resistance to Insecticides Develops in Fruit Flies Drosophila spp. in a Missouri Vineyard

Almost two years ago, I reported on the development of fruit flies developing cross-resistance to three insecticides in New York state. Fruit flies developed cross-resistance in a New York vineyard after three years of use of Mustang Maxx to control fruit flies. Fruit flies were collected from this vineyard after it was noticed that Mustang Maxx had failed to control fruit flies. Research determined that the fruit flies had developed resistance to three different chemical classes of insecticides (Table 1).

In March 2021, a Missouri Grape Grower reported that fruit flies (D. melanogaster) had developed cross-resistance to three different chemical classes of insecticides during the 2020 growing season. Similar to the New York fruit fly population, the Missouri fruit fly population developed resistance to three different chemical classes of insecticides (Table 1). The Missouri fruit fly population remains susceptible to Delegate (spinetoram) which is similar to the fruit fly population initially identified in New York.

Unlike the New York grower, the Missouri grower had a varied insecticide rotation plan. Insecticides were applied from three chemical classes (Table 2). However, within a given growing season there were two to three applications of Mustang Maxx (Table 2).

Table 1. Insecticides in which fruit flies Drosophila spp. have developed cross-resistance from a vineyard in Missouri in 2020 and New York state in 2018.

Table 2. Insecticide applications targeted to control fruit flies over four growing seasons in a Missouri vineyard.

¹ NA, no insecticide applied.
² Assail, acetamiprid applied for the control of Japanese beetles.

The Missouri fruit fly population may have experienced considerable insecticide selection pressure outside of the vineyard. Blackberries and peaches are established and producing fruit near some of the vineyard blocks. These fruiting plants flower and produce fruit well in advance of grape veraison and harvest. Both the blackberries and peaches received insecticide treatments for the control of Spotted wing Drosophila (SWD) D. suzukii mainly during the month of July. In addition, the growing areas of the blackberries and peaches had a considerable amount of cull fruit on the ground. The cull fruit would provide oviposition sites for both D. suzukii and D. melanogaster adult female fruit flies.

Insecticides are commonly used to control fruit flies in vineyards that have grape cultivars prone to the sour rot complex. The sour rot complex includes fruit flies, yeast, and acetic acid bacteria (acetobacter and gluconobacter). Yeast and acetic acid bacteria are ubiquitous within the vineyard. Fruit flies are needed for the spread and initiation of sour rot disease.

Additionally, a wound site is critical for the development of sour rot. The wound site may be split berries in tight clustered and thin skinned cultivars such as Vignoles, loss of berry skin integrity from a hail event or the separation of the berry from the pedicel and berry junction. A wound results in the yeast starting the fermentation of grape sugars to ethanol followed by acetic acid bacteria converting the ethanol to acetic acid. The yeast and acetic acid attract adult fruit flies. The fruit flies have some unidentified microorganism that plays a role in initiating sour rot disease. Therefore, adult fruit flies have been targeted for chemical control. Additionally, the incidence and severity of sour rot has been shown to be decreased by controlling fruit flies.

Moving forward grape growers should strongly consider and insect resistance management plan. This plan should include a fruit fly trapping and monitoring program. Fruit fly monitoring relies on establishing traps and keeping track of trap captures prior to beginning of an insecticide program. Followed by monitoring trap captures after an insecticide application. The idea behind the monitoring is to identify if fruit fly populations increase after an insecticide application. If fruit fly populations increase after a targeted spray application for controlling fruit flies this is a good indication that fruit flies have developed resistance to an insecticide. If you have relied almost exclusively on Mustang Maxx for fruit fly control over the past two growing seasons, then these vineyard blocks should be monitored closely.

Insecticides targeted for fruit fly control should be from different chemical classes. Do not rely on an insecticide from a single IRAC class for control. Rotate chemical classes and do not use the same chemical class of insecticide in repetition. Also be aware that insecticides applied targeting other insect pests may be putting selection pressure on fruit fly populations leading to the development of resistance. These may include Mustang Maxx or Assail applied for the control of Japanese beetle or Grape berry moth. As you write up an insect resistance management plan be sure to look back at previous year’s spray records.

In summary, a fruit fly population from a Missouri vineyard has developed cross-resistance to three chemical classes of insecticides The main insecticide applied for control of fruit flies over a four year period was Mustang Maxx. Although insecticides were rotated to different chemical classes since 2017, a fruit fly population still developed insecticide cross-resistance. Growers should implement an insect resistance management plan for fruit flies that includes a trapping and monitoring program, rotate insecticides from different chemical classes, and take into consideration how other insecticide applications targeting other insect pests may be impacting fruit fly populations, specifically selecting for insecticide-resistant fruit flies.

Grape Leaf Rust (Phakopsora euvitis)

The Missouri Department of Agriculture — Plant Pest Control Program has tentatively identified Grape leaf rust on grapevines located at retail outlet. This is the first report of Grapevine Leaf Rust that I am aware of for Missouri. The disease has been reported in southeastern states including North Carolina, South Carolina, Georgia and Florida. In 2018, Tennessee reported Grape Leaf Rust on grape seedlings located in retail outlets in six counties. Historically, Grape Leaf Rust can be considered a minor disease compared to other grape diseases including phomopsis, downey mildew, powdery mildew, black rot, and Anthracnose. The initial symptoms reported by the University of Tennessee appear on the upper leaf surface as small angular, yellow or brown lesions. On the lower leaf surface yellow-orange pustules (uredinia) contain spores (urediniospores) (Figure 1). As the disease progresses, the lesions coalesce and turn from yellow to brown/black. If the disease is severe leaf drop may occur. Grape Leaf Rust needs another host besides Vitis to complete its lifecycle. At this time the United States does not have the other host plant species present. With Vitis being the only plant species, the grape leaf rust continually produces urediniospores to reinfect grape tissue. Since grape leaf rust needs living green plant tissue to survive and reproduce it is unknown how Grape leaf rust overwinters in the United States. Some fungicides including Captan, mancozeb, and DMIs (Rally, Mettle, tebuconazole) should suppress Grape leaf rust according to the University of Tennessee. At this time Grape Leaf Rust in not a problem within Missouri commercial grape vineyards. Therefore, I am not advocating that any prophylactic fungicide applications be made for the management of Grape Leaf Rust.