What Is Wrong with My Plant? ◀ 15 In other diseases, several to many generations of secondary inoculum may be produced, resulting in widespread disease development. When secondary inoculum is produced, the potential for secondary disease cycles exists and a polycyclic epidemic (Fig. 1.10B) may result. Look back at the disease cycle shown in Figure 1.8 to see how it works. The time between infection by primary inoculum and the production of secondary inoculum is called the latent period. The terms incubation period and latent period have sometimes been used in confusing ways. However, it is important to differentiate between the interval from infection until the appearance of symptoms (incubation period) and the interval from infection until the production of secondary inoculum (latent period). Sometimes these intervals coincide, but they may be separated by several days or more.
M
Monocyclic Diseases
• Dutch elm disease • stinking smut • Verticillium wilt
M
Polycyclic Diseases
• apple scab • coffee rust • late blight of potato and tomato
Survival of Pathogens Annual plants die naturally at the end of the growing season or may die prematurely from disease or other causes. Pathogens of these plants must either produce survival structures or begin to function as saprophytes. Perennial plants may continue to harbor pathogens for years until their demise. But no plant is immortal, so even pathogens of perennial plants must find a new plant host eventually. The survival stage, when a pathogen is separated from its host, is a particularly vulnerable time for most biotic pathogens. This stage is very important because it is usually the source of primary inoculum that will lead to new disease outbreaks. CASE STUDY APPLE SCAB Disease cycles of biotic pathogens begin with primary inoculum that is dispersed to an infection court. If a disease is polycyclic, secondary inoculum will be produced, and additional infections may occur on the same host plant or other host plants during the growing season. The survival stage is often the source of primary inoculum that will initiate a new disease cycle.
Disease Management Using the Disease Cycle Management of a plant disease can focus on any of the important steps in the disease cycle: primary inoculum, infection courts, secondary inoculum, the dispersal of primary and secondary inoculum, and the survival stage. Approaches to disease management can be divided into four general categories: avoidance, exclusion, eradication, and protection. Avoidance of disease involves choosing a planting location where the pathogen is not present or choosing
Survival of the Pathogen In living leaves, the apple scab fungus is confined to the area between the host cuticle and the epidermis. Once infected leaves have fallen, the fungal mycelium colonizes them completely as a saprophyte. Venturia inaequalis usually overwinters in fallen leaves, surviving as mycelium and as early stages of sexual fruiting bodies (pseudothecia). These structures are the source of ascospores in the spring. Therefore, removal and destruction of fallen leaves is an effective way to reduce the disease. If leaves cannot be completely removed, as in a commercial orchard, protection of new leaves with fungicides, beginning at the time of bud break, is necessary in humid climates.