Louis S. HeslerUSDA-ARS North Central Agricultural Research Laboratory Marie A. C. LanghamSouth Dakota State UniversityThe incidence and severity of many plant diseases depend on organisms besides pathogens, hosts, and humans. Plant diseases may be facilitated by many different kinds of organisms serving as alternate hosts, vectors, or pre-disposing agents, while other organisms limit disease as pathogen antagonists. Several species of insects and other arthropods facilitate plant disease as vectors, pre-disposing agents, or both. Many of the most notorious insects that facilitate plant disease are aphids, whiteflies, scale insects, planthoppers and treehoppers—phloem-feeding insects that are part of a group known as the Homoptera. They generally infest leaves and stems. Homopterans have mouthparts that consist of four stylets used to pierce plant tissue and siphon plant sap. They can directly limit plant growth and reproduction through sap extraction and injection of toxins in their saliva.
Despite their direct damage from feeding on plants, homopterans are more significant in facilitating plant disease as vectors and pre-disposing agents. Table 1 compares the characteristics of two homopteran-dependent plant diseases, barley yellow dwarf and sooty molds, that are discussed below. Homopterans are the main vectors of viruses, spiroplasmas, phytoplasmas, and fastidious vascular organisms that cause plant disease. Homopterans vector these disease agents as they insert their stylets into plants. In the May 2004 News & Views, we discussed that feeding by cereal aphids can result in transmission of Barley yellow dwarf virus and Cereal yellow dwarf virus, two major viral pathogens that affect crops such as wheat and oats. These viruses can cause stunting, yellowing or similar discoloration of leaves, and yield loss in cereal plants. Barley yellow dwarf and cereal yellow dwarf may be managed by avoiding or controlling cereal aphids or by using plant cultivars that are resistant to the viruses.
Although homopterans can vector diseases through their feeding, they can also pre-dispose plants to disease by excretion of sugary waste called honeydew (Figure 1), which provides a suitable substrate for growth of ectoparasitic fungi that cause sooty mold. The fungi that cause sooty molds have high levels of melanin, causing them to have dark colored hyphae or spores and protecting the fungi from ultraviolet radiation. Sooty molds are most commonly caused by fungi in the genus, Capnodium, but can also be caused by fungi in the genera, Cladosporium, Aureobasidium, Antennariella, Limacinula, and Scorias.
Spores of sooty mold fungi are dispersed by wind and germinate after landing on honeydew-laden plant surfaces. The fungi grow as a superficial layer of dark mycelium that gives a sooty appearance to infected leaves and stems. Sooty mold reduces photosynthesis and becomes problematic when many homopterans feed and excrete large quantities of honeydew on a plant for substantial mycelial growth. When abundant, sooty mold reduces crop yield and lowers aesthetic value of ornamental plants causing problems for farmers, producers, horticulturists, and homeowners. The key to controlling sooty mold fungi is not to control the fungus, but to avoid or control the homopteran infestations producing the honeydew on which the fungi depend.
Table 1. COMPARISON OF TWO HOMOPTERAN-DEPENDENT PLANT DISEASES
Barley yellow dwarf
Pathogen type Virus
Role of homopteran in disease
Predisposing agent (various homopterans)
Pathogen location in or on plant?
In phloem tissue of cereal stems and leaves
On honeydew-laden crops and various plants
Disease severity in relation to aphid numbers
Only one viruliferous aphid is needed to infect a plant with virus, but virus trans-mission by multiple aphids generally increases disease incidence
Severity increases with amount of honeydew, which is directly related to number of aphids
Primarily avoidance or control of aphids; some cultivars of cereal plants have resistance to the virus
Primarily avoidance or control of aphids
For more information on sooty molds, try the following websites:
From the University of California, Pests in Landscapes and Gardens—Sooty Mold provides an overview of sooty molds with links to pictures of the homopteran insects involved in the complex.
Kenneth Kessler from the U. S. Forest Service discusses the basics of sooty mold identification and control on plants and other objects in his webpage, How to Recognize and Control Sooty Molds.
Stephen Nameth, Jim Chatfield, and David Shetlar of Ohio State Extension Services have information on sooty mold, including a listing of plants commonly affected by sooty mold in Sooty Molds on Trees and Shrubs.