Rusts have been acknowledged as diseases of plants almost since man has had a recorded history. The first records indicate that the Greeks and Romans knew of rusts and thought they were an expression of the wrath of God. The Romans created a special rust god, Robigo, and held a festival each year in which offerings were sacrificed to appease Robigo, so he would not destroy their grain and other crops.
The name rust comes from the fact that some rust fungi make the plant tissues look rusty. There are many species of rusts, which attack almost every plant family on earth. The most notorious rust disease epidemics have been on grain crops - wheat, oats, barley, and safflower. But many vegetables are also affected. Beans, peas, asparagus, garlic, carrots, cowpeas, and even lettuce are examples. Ornamental crops have their share of rust diseases as well - snapdragon, carnation, artemisia, rose, chrysanthemum, delphinium, hollyhock, and geranium - to name a few. Finally, fruit trees and vines like pear, apple, peach, grapes, and plum, all have rust pathogens, which can cause severe disease if not controlled.
Rust diseases are often first evident as small, yellowing spots on the upper surface of the leaf. Some rusts do not cause this color change and are only discovered when the underside of the leaf is examined. Evidence of infection is then seen as a blister, which may be yellowish to reddish gray, depending on the host and the species of rust. The blisters (host epidermis tissue) mature and rupture to reveal a mass of spores, which may be yellow, orange, brick-red, or brown. These spores are powder-like and are easily airborne to new infection sites on the same plant or to down wind sites. The spores will germinate and cause new infections if free moisture is present for several hours and the temperature is right. This cycle can be repeated several times a season if temperature and moisture conditions remain favorable. When conditions become unfavorable, many rusts develop a survival spore or black spore stage, which can live in dead plant refuse. Some rusts have more than one host and will spend the winter on a perennial host and the summer on an annual host. When rusts change seasonal stages or host stages, they often also change color and appearance. In asparagus rust, which is a single host rust, the several spore stages may be present on the same plant and be orange, brick-red, and black, while also exhibiting a different morphology.
When devising control strategies for rust, it is important to understand whether the rust is a single host (autoecious) or a multiple host rust (heteroecious). Control of an autoecious rust involves only the immediate affected plants. Control of a heteroecious rust will involve two hosts and their proximity to each other. Depending on their economic value, the alternate host may be eliminated or it may be possible to separate them spatially, so there is less chance of spore exchange. Where moisture control is possible, keeping leaf surfaces dry is helpful in suppressing rust development.
Besides host and environmental management, most rusts must also be suppressed by the use of plant protection products. Systemic fungicides, such as triadimefon (Bayleton®), triforine (Funginex®), myclobutanil (Rally®, Nova®), thiophanate-methyl (Topsin M®), and propiconazole (Tilt®) are usually most effective, However, contact fungicides, such as tebuconazole (Folicur®), chlorothalonil (Bravo®, Daconil®), maneb, mancozeb, thiram, copper fungicides, captan, ferbam, ziram, and sulfur should not be overlooked as possible control agents. Often times, label restrictions are such that a lesser efficacious product be used to help keep rusts from severely damaging a crop.
Whatever fungicide is used, preventive treatment and treatment at the earliest stages of disease detection is useful in keeping disease incidence at low levels. Always check the product label for registered uses and for rates and timeliness of application for most efficacious control of the problem at hand. Contact your Cooperative Extension advisor or the chemical manufacturer if you have questions.
December 1999 issue of this newsletter I published a short article on African Bees. We anticipate that the African bee will be colonizing eastern Santa Barbara County (Cuyama Valley and surrounding area), and the coastal region of southern Santa Barbara County (Carpinteria to Point Conception) during the spring/summer of 2000.