We can grow nearly anything year-round in Hawaii, but when we lose the ability to grow plants because of diseases or pests, some of the joy of gardening is lost. We can grow nearly anything year-round in Hawaii, but when
We can grow nearly anything year-round in Hawaii, but when we lose the ability to grow plants because of diseases or pests, some of the joy of gardening is lost.
While some pests fly, crawl or float into our gardens, many more are inadvertently invited. How often do you check plants you purchase, or take from a friend for unwanted pests or diseases that may be hiding in the soil, on or in the plant?
As the adage goes, “The best way to keep an unwanted guest out of your house is not to invite them in the first place.” These pests and diseases can be very sneaky and hide very well, only to show themselves after there is no hope of correcting the problems short of a lava flow. While this example may seem a bit dramatic, it is all too real for some who deal with black rot or bacterial wilt.
The best way to keep seed borne diseases out of your garden is to plant disease-free seeds. Many of the reputable commercial seed companies produce seeds in areas that are free of disease or are grown in areas not conducive to disease development. They test their seeds for diseases and conduct seed treatments as necessary. When obtaining seeds from friends or seed exchanges, it is always good to ask if the parent plant was free of disease. It is also a good idea to look at the seeds to see if anything looks out of place. Quarantine questionable plants if you have to have them.
To understand seed borne disease, it is important to understand how seeds are infected in the first place and why it is impossible to clean some of them. Infection normally occurs during the seed development process, anywhere from the time of flowering to the time of seed harvest. Virus infection may result from the parent plant being infected and the virus settling in the developing ovary, such as the case with lettuce mosaic virus. Bacteria and fungi can also infect plants early in the seed development process and become embedded within the embryo or the seed coat. Other infections occur after the seeds are partially developed but before seed maturity and harvest. These infections are generally found in or on the seed coat. In a few cases, infection occurs after seed maturity, normally during wet seed production periods.
Whether an organism can be removed from infected seeds depends on numerous factors including the disease, seed type and infection severity. Disease organisms that adhere to the seed coat are the easiest to eliminate, through surface sterilization, washing and physical removal. Those that are embedded within the seed coat or the cotyledons are harder to deal with. In these cases, it usually comes down to differential survival of the disease organism and the seed. For long-lived seeds such as celery, the control of septoria late blight is to store the seeds for a year or two during which the disease organism dies. Other treatments take advantage of differences in heat tolerance. Many of the cole crops, cabbage and its close relatives, are treated for the bacterial black rot by soaking them in hot water for 30 minutes. For the treatment to be successful, precise procedures must be followed. The water temperature must be correct, as well as the duration of soaking. Higher vigor seeds will better withstand heat treatment.
If you collect seeds from your garden, it is good practice to follow basic rules to reduce incidents of disease. Eliminating diseased plants is the first step. Seasons and locations with more sunshine, less rain and consistent breezes are better for seed production, as these conditions are unfavorable for disease development. Do not leave matured seeds in the garden where disease organisms can grow on them. Harvest them immediately and place seeds in a dry, shady location preferably with air movement to aid the drying process.
Vegetatively propagated plants such as potato, taro, cassava and banana can also host a number of diseases. Cuttings, corms and taro huli can all harbor viruses, bacteria and fungi that can easily spread within your garden once introduced. We should all expand the number and types of plants we grow in our gardens, but we should also be vigilant about introducing unwanted guest.
For more information on this and other gardening topics, visit the CTAHR electronic publication website at ctahr.hawaii.edu/Site/Info.aspx or visit any cooperative extension service office islandwide.
Nagata can be reached at russelln@hawaii.edu.
