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Rust

Rust diseases are characterized by a rust colored discoloration on the leaves and fruit, often affecting apples. They are caused by different species of the fungus Gymnosporangium and have various junipers and red cedars (juniperus species) as an alternate host. Apples are generally most susceptible to infection by the rust fungi during the period from early bloom until about 30 days after bloom.

Symptoms

Cedar-Apple Rust on Apple

On leaves, pale yellow spots appear on the upper surface during May or June. Spots are up to l/4 inch in diameter, turn orange with time, and often have a reddish border. Small black fungal bodies (pycnia) form within the spots and may exude an orange fluid. In time, yellow spots develop on the underside of the leaf. These spots thicken, and during late spring and early summer a number of small, orange-yellow tubular projections (aecia) appear. These develop into open, cylindrical tubes that split toward the base into narrow strips and curl backward. Infected leaves may turn yellow and drop. Defoliation of rusted leaves is most common in dry summers. On fruit, similar yellow-orange spots appear, usually at or near the calyx end. These spots usually occur on immature fruit and are much larger than the spots on leaves (up to 3/4 inch in diameter). The light green color of the young fruit becomes a darker green around the infected area. The tube-like aecia may form on the slightly raised fruit lesions. Infected fruits are often stunted and misshapen, and may drop early.

Cedar- Quince Rust on Apple

Cedar-quince rust only affects the fruit of apples. Infected fruit become puckered at the blossom end and later develop a sunken, dark green area. The flesh under the sunken, dark green area becomes brown and spongy. The formation of pycnia and aecia on infected fruit is rare. Apples are susceptible to cedar-quince rust during the period from early bloom through third cover. Cedar-Hawthorn Rust on Apple Leaf spots similar to those caused by cedar-apple rust develop on apple and crabapple. Larger, gray to brown spots form on leaves of hawthorn. Few aecia form on apple and crabapple. Fruit infection on apple is rare. Defoliation and deformation of fruits and twigs may occur on hawthorns.

Causal Organisms

Cedar-apple rust is caused by the fungus Gymnosporangium juniperi-virginianae, cedar-quince rust by Gymnosporangium clavipes, and cedar-hawthorn rust by Gymnosporangium globosum. The disease cycle for all three rusts is essentially the same. The disease cycle for cedar-apple rust will be presented. The fungus overwinters as mycelium in galls in juniper. Large yellow to orange gelatinous sporehorns are formed on the galls in the spring and spores (teliospores) are produced. Each teliospore germinates and produces four to eight sporidia or basidiosproes. As sporehorns begin to dry, the sporidia are forcibly discharged into the air and carried by wind to nearby apple leaves, fruits, and twigs. About 30 days after apples have bloomed, the sporehorns have discharged all their spores and most apple leaves are no longer susceptible. Within five or six hours after landing on the leaf, sporidia become attached to the surface, germinate, and penetrate the cuticle and upper leaf surface. After 10 to 14 days, the yellow spots develop on the upper leaf surface. The orange to black pycnia develop in the spots and several weeks later, the aecia form on the underleaf surface. The aecia produce another type of spore (aeciospores) that are carried by wind to junipers. When the aeciospores contact a juniper twig, they become firmly attached and germinate in warm moist weather of late summer or early fall and penetrate the twig. A young, pea-size, greenish-brown gall develops. The gall enlarges the following year, but does not produce sporehorns with teliospores until the second spring. The complete disease cycle requires almost two years.

Control

  1. Grow resistant or immune apples, crabapples, and junipers. When buying trees, check with the nursery about rust resistance.
  2. Destroy nearby, worthless or wild junipers infected with rust galls.
  3. Where rusts are a problem, follow a recommended fungicide spray program.

Asparagus

Asparagus rust is a fungus disease that attacks fern growth, reducing the amount of photosynthesis and food translocated to the crown for next year's crop. The 'Washington' types available today are susceptible to rust. The high level of rust resistance present in the original 'Washington' types has been lost over the years. The New Jersey hybrids have tolerance to rust, but not resistance.

On the fern, rust symptoms appear first as brick-red pustules (uredia) that develop from late spring through the middle of August. These uredia produce spores (urediospores) that are blown to neighboring plants where they initiate new infections. Infection by urediospores is favored by periods of rainfall or heavy dew. In mid-August, the pustules change color from red to black and are then called "telia" (Figure 22). The spores produced in these telia (teliospores) cannot infect plants in the same season, but are able to survive during the winter, attached to fern debris, and provide inoculum for the following spring.

To remove the source of inoculum, the old ferns can be burned in the early spring before spear production starts. However, any spores present on spears are physically removed during harvest. Infections on the ferns can come later from spores blowing into the field.

Rust fungi produces spores which appears as reddish-orange spots on the leaves of affected plants. These areas of orange are easily rubbed off when brushed. They can often cause great damage to some crops.

An organic approach to the problem is dusting the leaves with lime.

Asparagus rust appears as reddish or brownish spots on the stems and leaves of Asparagus. The spots release a cloud of spores when brushed or rubbed. The disease will certainly cause poor quality shoots and often will kill the plant. The problem usually occurs in damp areas, because the spores need moisture to germinate.

Plant resistant varieties, such as Mary Washington, in a well drained area to prevent the problem. Control the disease with applications of a fungicide, such as Zineb or chlorothalonil. Wet media and spray with elder leaves. Can also be controlled by drying out growing area, reducing humidity. Wet media and spray with elder leaves. Can also be controlled by drying out growing area, reducing humidity.

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