The Five Stages of the Disease Cycle
These are the five stages of the disease cycle:
Inoculation: The weed seed, pathogen, or insect egg has to come into the lawn area. Scientists call anything that carries a pest problem a vector. It could be the wind, a bird carrying weed seeds in its crop, or adult insects arriving from the neighbor's yard. The wind could be a vector if it shares dandelion seed from yard to yard or blows small, weak-flying insects from yard to yard.
Prepenetration: Where specific conditions exist that are beneficial for the pest to become established, the pest has to start to grow before it can influence the grass plants. For example, some bacteria can start to grow in the soil that will attack the grass plant. If conditions exist to allow the bacteria to grow, they will survive and prosper.
However, if conditions are not good—if there is competition for the food source or soil chemicals (compost provides both of these things) that kill the bacteria are present-the bacteria will not survive. Insect pests need a place to grow and develop. For example, if the soil population of beneficial nematodes is low because of chemical sprays, then flea beetles will be able to lay eggs.
Penetration: This is the stage when the pests physically attack the grass plants. The bacteria gain entrance through a wound like a ragged mowing cut (from dull blades) or the insect pests start to have lunch.
Infection: This stage is immediately after penetration when the pest population starts to feed off the individual plants or successfully compete with them for nutrients.
Some infections are immediate and the effects will show for all to see, while others are latent. Latent infections exist in the lawn but are partially suppressed by the lawn's natural defenses. It takes a bit of stress on the grass plants (such as drought or overfeeding) for the infection to explode fully and make its presence felt to the homeowner. For example, if Diazinon is sprayed to eliminate lawn grubs, it will kill off most of the spider mites and their predators as well. Spider mites rebound much quicker than their predators, so experienced lawnowners check for an explosion of spider mites after they have sprayed Diazinon for grub control.
Propagation: Once the pest has established itself and started to feed, the next step is to reproduce, find a vector to take it on a journey, and replicate its success in some other yard or area of your yard.
The important point about lawn or grass plant health is that the homeowner can stop or attack plant pests at any one of these five stages. Remember pests are insects, diseases, or anything that negatively influences grass. Once you know the pest, you can figure out the easiest way to control it for the long term.
Prepenetration work is the preventive work in the garden that hinders pests. It is here that average gardeners can stop the majority of problems they are ever likely to see. In simple terms, keep the grass healthy and the pests can't become established.
You'll often see the term LD50 in technical literature about gardening chemicals. This stands for Lethal Dose 50 percent. In short, it is the amount of a chemical it takes to kill 50 percent of the test population of animals. These tests are most often done on rats. Although the chemical is measured based on the ratio of the chemical to the weight of the rat, the bottom line is still when half the rats are dead, that's the LD50.
“LD” is a good relative measurement in that we can see how one chemical stacks up against another. We know that there are ranges above which garden chemicals are relatively safe and below which they are considered quite dangerous to use.
|High toxicity—dangerous||Oral: less than 50||CleanCrop PMA LD50 = 22|
|Fatal at 1 drop to ½ teaspoon||Diazinon LD50 = 108|
|Dermal (skin): less than 200||Dursban LD 50 = 135|
|Medium toxicity—medium||Oral 50 to 500||Sevin LD50 = 400–850|
|Fatal at ½ teaspoon to 2 tablespoons||Banner LD50 =1310|
|Dermal (skin): 200–2,000|
|Low toxicity—low||Oral: greater than 500||Insecticidal soap LD50 = greater|
|Not fatal unless over 1 ounce||than 10,000|
|ingested and then variable||Roundup LD50 = 4320|
|Dermal (skin): greater than 2,000|
So, while there are potential problems using any garden chemical, the LD50 rating is a good general guideline to immediate dangers. Most home gardening product labels do not list the LD50 rating. However, they do categorize themselves in the following manner:
If the LD50 rating is high toxicity, the label will say, “Danger—Poison.”
If the LD50 is medium toxicity, the label will say, “Warning.”
If the LD50 is of low toxicity, the label will say, “Caution.”