Diseases caused by oomycetous fungi of the order Peronosporales present major problems worldwide. Important foliar diseases include late blight on potatoes, blue mold on tobacco, grape downy mildew, plus a wide range of other foliar blights and downy mildews on cereals, fruits, vegetables, and ornamentals. Soilborne Phytophthora and Pythium spp. are equally wide­ spread and are the cause of major losses on crops as diverse as soybeans (148) and avocados (33, 51). In addition, Phytophthora and Pythium spp. are responsible for many preand postharvest problems on fruits and vegetables, including late blight of potato tubers (10), brown rot of citrus (35-37, 94), and black pod of cocoa (125, 126). Less than a decade ago our means of adequately controlling many of these diseases was extremely limited. Control of potato late blight and downy mildews involved repeated applications of protectant fungicides, starting well in advance of the general appearance of the disease. The control of many soilborne pathogens was even more limited. Fumigation with methyl bromide gas, or soil drenches with nematicides, offered only limited control for some high-value annual crops. Soilborne pathogens of field and perennial tree crops were not controlled by chemical methods, because of either cost or lack of efficacy of available chemicals. However, the discovery of systemic fungicides with good activity against Oomycetes in the early 1970s continues to revolutionize our concepts of

Systemic Fungicides and the Control of Oomycetes

Novel disease control compounds: the potential to ‘immunize’ plants against infection

Mixtures of herbicides have been proposed as strategies to prevent or delay the evolution of resistance to the resistance-prone sulfonylurea and imidazolinone herbicides that inhibit acetolactate synthase. These herbicides have become or are becoming widely used in soybean, wheat, rice, and other major crops. For a mixture to be efficacious in preventing resistance, the less resistance-prone component(s) should have the following traits compared to the vulnerable herbicide: a) control the same spectra of weeds; b) have the same persistence; c) have a different target site; d) be degraded in a different manner; and e) preferably exert negative cross-resistance. We compared the proposed mixing partners for use with several widely used acetolactate synthase inhibiting herbicides to these criteria and found that: a) all have somewhat different weed spectra; e.g. none control common cocklebur as well as imazaquin or imazethapyr in soybean, or kochia as well as chlorsulfuron in winter wheat; b) all are far less persistent than these vulnerable herbicides. Less persistent sulfonylureas are now on the market but are in limited use. Late in the season, the mixing partner is not present while the vulnerable herbicide remains active; c) most have different target sites; d) in soybean most mixing partners are degraded differently than vulnerable herbicides. In wheat virtually all herbicides used without safeners are degraded by monooxygenases, thus it is impossible to meet this criterion in this crop; e) none of the mixing partners exert negative cross-resistance. The present mixtures may have superior or more cost-effective weed control properties than the acetolactate synthase inhibitors used alone, but they do not meet all the criteria for resistance management. Not meeting the key criteria of identical control spectra and equal persistence aggravates future resistance problems, as has happened with insecticides.

Are Herbicide mixtures useful for Delaying the Rapid Evolution of Resistance? a Case Study

This paper reviews the evidence relating to the question: does the risk of fungicide resistance increase or decrease with dose? The development of fungicide resistance progresses through three key phases. During the 'emergence phase' the resistant strain has to arise through mutation and invasion. During the subsequent 'selection phase', the resistant strain is present in the pathogen population and the fraction of the pathogen population carrying the resistance increases due to the selection pressure caused by the fungicide. During the final phase of 'adjustment', the dose or choice of fungicide may need to be changed to maintain effective control over a pathogen population where resistance has developed to intermediate levels. Emergence phase: no experimental publications and only one model study report on the emergence phase, and we conclude that work in this area is needed. Selection phase: all the published experimental work, and virtually all model studies, relate to the selection phase. Seven peer reviewed and four non-peer reviewed publications report experimental evidence. All show increased selection for fungicide resistance with increased fungicide dose, except for one peer reviewed publication that does not detect any selection irrespective of dose and one conference proceedings publication which claims evidence for increased selection at a lower dose. In the mathematical models published, no evidence has been found that a lower dose could lead to a higher risk of fungicide resistance selection. We discuss areas of the dose rate debate that need further study. These include further work on pathogen-fungicide combinations where the pathogen develops partial resistance to the fungicide and work on the emergence phase.

https://bsppjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3059.2011.02439.x

The dose rate debate: Does the risk of fungicide resistance increase or decrease with dose?

Fungicide-resistance management would be more effective if principles governing the selection of resistant strains could be determined and validated. Such principles could then be used to predict whether a proposed change to a fungicide application program would decrease selection for resistant strains. In this review, we assess a governing principle that appears to have good predictive power. The principle states that reducing the product of the selection coefficient (defined as the difference between the per capita rate of increase of the sensitive and resistant strains) and the exposure time of the pathogen to the fungicide reduces the selection for resistance. We show that observations as well as modeling studies agree with the predicted effect (i.e., that a specific change to a fungicide program increased or decreased selection or was broadly neutral in its effect on selection) in 84% of the cases and that only 5% of the experimental results contradict predictions. We argue that the selection coefficient and exposure time principle can guide the development of resistance management tactics.

W. J. Houser

Papers

Reduced-rate fungicide mixtures to delay fungicide resistance and to control selected turfgrass diseases

Control of Pythium spp. and Pythium blight of turfgrass with fosetyl aluminum