Basic Plant Pathology

A Basic Course in Plant Pathology

CRFG Fruit Gardener V0l. 22 No. 4 1990

Notes on a Talk by Professor Robert Raabe
Department of Plant Pathology
University of California at Berkeley
Reported by MeIita Israel, Monterey Bay Chapter

Dr. Raabe, an expert on diseases of ornamentals, started by explaining there are many crossover diseases that affect both ornamentals and fruit trees. He divides diseases into infectious and non-infectious.

Non-infectious diseases are those that result from a plant’s reaction to its environment. These symptoms do not spread as infectious diseases do.

Lack of iron in peach trees is discernible because the young leaves are pale yellow. Iron does not circulate in a plant. Old leaves can have iron, but this iron will not go to the young growth. Usually the iron deficiency shows up first at the margins of the young leaves. The veins of the leaf are not affected and are green. It is the tissue between the veins that is pale. California soils are usually high in iron, but it is tied up by calcium ions (high pH) in the soil and not available to the plant. To correct an iron deficiency the plant can be sprayed with iron chelate. If iron is added to the soil it will leach out. The spray is a temporary solution. To permanently correct the problem, add as much organic matter to the soil as possible, preferably before planting. This will make the soil more acid and the iron in the soil will be released for the plant to absorb. Sulfur can also be added to make the soil acid, but this also leaches from the soil. Adding organic matter is the more permanent solution.

Manganese deficiency looks similar to iron deficiency. However the pale color is in the older growth.

Zinc deficiency is called little leaf. Leaves do not grow as large as they should. Color loss is not consistent. To correct the problem, hammer zinc coated window glazing points into the tree trunk. These are small and do little injury to the tree’s trunk.

Fertilizer excesses (salt damage) tend to kill the tissue at the margin of the older leaves. Stop fertilizing and water the trees as much as possible without keeping the soil excessively wet.

Sunburn bleaches and kills the tissue between the veins. Color is variable. To correct the problem be sure the plant has sufficient water. Plants without chlorophyl tend to sunburn more easily. Leaves affected with mosaic virus that has depleted the chlorophyl will sunburn most easily.

Toxic substances applied to the soil, such as weed killers, cause leaf discoloration.

Trees can be killed by raising the soil around the trunk. It is believed that this stops an air exchange at the crown provided by the bark of the tree.

Lack of winter chill can cause leaf problems. The older leaves don’t fall; and the tree does not get its proper rest. In Israel they are wetting trees to increase the winter-chill factor. They found that warm spells, during winter, negate some of the chilling time needed by certain fruit trees.

Many diseases affecting trees are found in the soil, especially clay soils where the water may stay and block the air exchange needed by the roots.

Bacteria have plasmids that disseminate and move through the plant. Scientists have found by knocking off certain genes in the plasmid a new gene can be added that alters the plant’s gene makeup. By adding bacillus thuringiensis to the gene makeup, the plant tissue will kill caterpillars. However scientists are now finding that certain caterpillars are not affected by the bacillus thuringiensis.

Scientists are also developing crops that are resistant to Round Up. This will allow fields to be sprayed for weeds without killing the crop.

Crown gall infects the roots at the soil line. It is a brain-like mass of tissue caused by bacteria. Crown gall can be found deeper in the soil on tree roots. The tree usually does not die, but is stunted by crown gall. To kill the gall, paint it with kerosene. This doesn’t work in all cases, but is worth trying when you can see the gall at the crown.

Another root gall is caused by the woolly apple aphid. This aphid only attacks the Ma/us genus including pyracantha as well as apples. There is swelling on the roots as well as limbs. These insects excrete a waxy substance that can be washed off with water. This insect does not fly. A strip of Tanglefoot on a paper collar can trap the insects on the sticky surface and help control this problem.

Hairy root is caused by bacteria. It was found that infecting a fruit tree with the hairy root bacteria in cold climates, such as in Montana, helped new trees become established much sooner in the shorter growing season.

Fireblight enters the blossom and kills the tissue very rapidly. Pears are most susceptible to this disease which also affects the Ma/us genus. The chief carrier are bees that transport the bacterium from flower to flower. If you catch this disease in the pink stage and cut off the limb about 6 to 8 inches below any sign of the disease, it can be controlled quite well. Pyracantha is a good host. If your trees are affected with fireblight it will help if the pyracantha is removed.

Another disease is called blast or in stone fruits it is called canker. Deep fissures exude a pitch, although some trees will do this naturally. This deep wound needs to be pruned out. If the canker spreads around the bark the limb will die. If located at the trunk of the tree, the tree will die when the root system decays. The inner bark, the cambium layer, is often reddish-brown when infected with this disease. Best control is to have good drainage and not allow watering on the trunk. Diseased trees should be removed and burned. Canker-producing bacteria overwinter in the margins of the cankers, so cut to good tissue.

Citrus trees also get blast if not grown well. A sign of this disease is gummosis. No effective control is known. Good drainage helps prevent this disease. Do not overwater.

Oak root fungus can usually, but not always, be seen as a white mat. It smells like mushrooms and makes your mouth water. This fungus is found at the crown of the tree and tree roots. It does not grow through the soil but must have actual root contact. It stays in the soil for many years. The best control is to plant rootstocks that are resistant to the disease. Good drainage helps.

Verticillium wilt, which attacks many woody plants such as pistachio, olive and some stone fruits, causes dieback. If you peel back the bark there is a discoloration of the cambium layer. The disease stays in the soil at least 20 years. Don’t plant trees in soil that has grown strawberries, tomatoes or cotton. Trees are infected by verticillium through the roots. The fungus plugs the water-carrying tissues. Crop rotation helps control the disease. No sprays are effective in control of this wilt.

Damping-off is a water mold caused by fungi. It collapses the seedling or rots the seed. To control, don’t plant seeds too deep or too close together. Don’t overwater or fertilize too much. The more succulent the growth the more the chance of decline. Seeds can be treated by placing a pinch of powdered fungicide in the seed packet. The pink coating found on treated seeds is relatively innocuous to us, but don’t eat it.

Use pasteurized soil to start seeds. Place a thermometer in a two-inch layer of soil on a cookie-sheet and heat for 30 minutes at l400F, cracking the oven door open to keep temperature at l400 . This leaves the beneficial soil contents. Do not microwave soil. Be sure to use clean flats or pots to prevent damp-off.

Powdery mildew is caused by lack of rain with warm days and cool nights and high humidity. It attacks new foliage on apples, berries and grapes. Tiny flecks peg into the tissue. Sulfur sprays help. It helps if you water the plants overhead early in the morning so the plant can dry before nightfall.

Black sooty mold lives on the excreted sugars, that sucking insects produce. It is unsightly and can be washed off. This fungus doesn’t hurt the leaves. The insects causing the problem need to be controlled usually with soapy water or oil sprays.

Black spots on the leaves of strawberries, loquat, quince, evergreen pear and Raphiolepis is caused by a fungus that is water-dispersed. There is no control.

Scab fungus on apple and loquat has no control. This superficial injury stops the development of tissue. Benolate may help. Benolate can help pear scab.

Peach and nectarine leaf curl is caused by fungi that live through the dry weather as spores. To control, spray with lime sulfur or wettable copper just before buds open. If buds open you have lost control.

Brown rot affects stone fruit through the blossoms. Apricots are most affected. The fruit rots before it ripens. The use of benolate and oil in winter and benolate after flowering, helps control rot. Commercially fruit is given a bath containing dilute benolate to keep fruit from rotting. It is most important to wash purchased fruit from the grocery store before eating it.

Do not add rocks or broken pottery to pots. This causes poor drainage. It blocks air to the roots.

Study where your plant came from. If it came from a Mediterranean climate, it will prefer no water during the summer. Check on the weather of the country and the rain pattern. Do what the plant likes best.