Chapter 30: Threats to agriculture: insects and pathogens
Chapter 30: Threats to agriculture: insects and pathogens
As any farmer will tell you, there are many threats to growing plants. Some of these have been covered in earlier chapters: extremes in temperature and moisture (Chapter 26); and interactions with other plants, i.e. weeds (Chapter 29). A large number of additional threats come from trophic interactions with a vast diversity of grazers, parasites and and predators. Certainly the most familiar of these are insects but other invertebrates (mites, nematodes, mollusks) and vertebrates can be significant obstacles to growing plants. Moreover, a large number of fungi, bacteria (Fig. 1) , protists and viruses are also be important threats.
TOPICS
- Grazing herbivores
- Absorbing herbivores
- Plant defenses
- Boundary
- Chemicals
- Phenology
- A sampling of plant disease
- Phytoplasmas and defining disease
- Late blight of potato—the disease triangle
- Wheat rust disease—complex pathogen life cycles
- Corn smut disease—Mexican truffles
- Dutch elm disease —overreaction can kill you
- Rice blast disease—changing strategies
- Fire blight of apple—bacterial phytopathogens
- Crown gall disease—making a disease a tool
- Viral diseases—tobacco mosiac virus, cauliflower mosaic virus, ring spot disease of papaya
Grazing herbivores
Most animal herbivores, e.g. woodchucks and adult japanese beetles, are considered grazers, eating parts of the plant and using their mobility to get to other plants. However, a substantial number of insect herbivores have feeding stages (generally larvae, e.g. leaf miners, emerald ash borers, larval potato beetles) that are immobile to the extent of getting to new host plants. These might better be considered parasites because they feed solely on one host plant. Most non-animal herbivores (fungi, bacteria) are basically immobile and are considered parasites although some are capable of grow ing from one host to another and many are transported from one plant to another by wind, rain and insects.
Most grazers on plants are ingesters (see Chapter 27), they remove and consume part of the plant, shred it to some extent and then digest the pieces in an internal tube, and ultimately excrete undigestible to their environment. Grazers, although they can severely reduce crop yields, rarely completely kill the plant it is eating both because they tend to specialize on only part of the plant (most commonly leaves) and because the plant is capable of replacing the organs/parts of organs that have been lost.
The plant material acquired by grazers is generally not very nutrient dense but varies substantially depending upon the part of the plant eaten. Nutritional quality is largely influenced by how many living cells are present (dead cells are basically all cell walls and are indigestible for most herbivores). Most herbivores specialize on one of the tissues listed below, but there are some generalists that feed on multiple tissues, e.g. browsers like moose and deer often eat stems, leaves a