As far as late blight is concerned, I believe the two most important measures of prevention were quality, uninfected seed, and rotation. I've saved my own tomato seed for three generations, and each year I follow the method of fermenting the seed pulp before washing the seed and properly drying it for storage. The fermentation process promotes good germination, but it also destroys potential pathogens that can be harbored by the seed. Our potato seeds are put to the test by coming through a long storage period, proving that they are disease free.
Second to quality seed in disease prevention, is rotation. It is best to maintain a 2-3 year rotation of crops, and whenever you are introduced to a new disease in your garden, it is important to research the alternate hosts of this disease. It can be surprising to find dissimilar crops can host the same disease, and often weeds will harbor and spread disease. Along with rotation, it is necessary to remove any plant residue (leaves, stems, roots) from the garden and properly compost the material before returning it to the soil.
These two important factors certainly contributed to our garden's blight free status, despite the prevalence of blighted potato fields no more than 5 miles from us. Healthy plants will be able to withstand certain thresholds of disease, borne in on the wind and rain, so part of the story is plain good fortune that our crops did not succumb. But keeping the plants from contact with the soil can also prevent or suppress blight. So perhaps the layer of mulch we applied to the crops in mid-June hampered the incubation process, necessary to the spread of the disease.
I always thought of carrots as a rather trouble free crop, once they are thinned and weeded, they generally take care of themselves. We've had some damage from Carrot Root Fly (or Carrot Rust Fly), but again, rotation and fall cultivation is usually enough to keep their numbers under control. This year I have been introduced to a fungal disease that affects the carrot leaves, Alternaria Leaf Blight. It affects older crops, later in the season after the rows have closed in, reducing air flow. Like other fungi, it thrives in cool wet conditions. But where did it come from, how was it introduced to our garden?
Going back to my original statement, about the importance of quality seed, I realized that my first carrot seed saving venture was flawed. I had selected quality storage carrots to grow for seed last summer, but I had not read enough information about preventing the spread of disease through seed. Alternaria is most often transmitted through infected seed. And there is a simple method of treating seed to halt it's spread to the next crop. Many types of seeds, including the Cole family, lettuce, spinach, eggplant and nightshades, as well as carrots, can be treated in a hot water bath, for a specific length of time (see the table on this link for details). I have also read about using a hot water and cider vinegar solution, though it looks as though the temperature and length of exposure is the most effective measure of killing potential pathogens.
The Alternaria leaf blight does not much damage the crop, especially at the later stages when most of the growth is complete. For the most part, it is a problem for mechanical harvesters, as the stems break off easily, leaving roots in the ground. The most damaging effect of this fungus is that is promotes the spread of damping-off in the soil. And I do believe that my last planting of carrots was cut back by damping-off. This could lead to potential problems for direct seeded crops and young seedlings in the future.
Cereal crops have their own biosphere of diseases, including fungi, bacteria and viruses. Most of these disease cycles can be broken by planting non-cereal crops in rotation, but some vegetable and legume crops can host cereal diseases as well. Developing resistant seed in cereal crops is an important and active area of agricultural research. Research labs around the world are constantly responding to new mutations of cereal diseases, breeding crops with genetic resistance, and making these resistant varieties available to many Third World countries whose farmers do not depend on fungicides for healthy crops.
We bought our Vicar Hulless Oat seed from a small independent seed producer, the seed is an heirloom variety, and granted, there are few options for purchasing small quantities of cereal seeds in Canada, so we hoped for the best. The description of the seed said nothing of disease resistance, and in the future, that will be a requirement in my seed purchases. Our Hard Red Spring Wheat is a modern, disease-resistant cultivar, proven in our climate, and it has withstood the variety of cereal diseases this year. The Hulless Oats, on the other hand, are peppered with Septoria or Speckled Leaf Blotch, a fungal disease. It can easily spread to other cereal crops, so our healthy crop of wheat, only 20 feet from the infected oats, has given me first-hand evidence of the necessity of disease resistant cultivars.