Plant Diseases Plant diseases reduce the harvest of food worldwide by about 30% each year. Diseases can take many forms, and cause many different types of symptoms in host plants. Ex: necrosis (death), wilts, loss of fruit, economic damage to harvested crops

Tomato early blight (Alternaria solani) -caused by a soil-borne fungus, can over winter in plant debris on the ground. -causes dark sunken lesions on mature fruit, concentric rings of necrosis, fruit often falls from the plant before harvest. -also causes defoliation of the host plant

Tomato Early Blight, Altenaria solani

2) Grey Mold (Botrytis cinerea) caused by a soil-borne fungus, easily invades new tissue on many plants. -requires high humidity, close proximity of other plants. -attacks stems first, causes black slimy lesions, which cause the plant to die. -lesions on fruit cause economic loss.

Grey Mold of bean, (Botrytis cinerea)

Botrytis mold on raspberries and strawberries

3) Corn Smut (Ustilago zeae) -overwinters in soil as spores, which can survive in soil for over 3 years. -causes ears of corn to turn black, with a greasy appearance. Large amounts of spores are produced within the infected ears. -these spores can be blown on the wind to other plants.

Corn smut, Ustilago zeae

-one of the most devastating plant diseases, it is responsible for the 4) Potato Late Blight, (Phytophthora infestans) -one of the most devastating plant diseases, it is responsible for the Irish Potato Famine of the 1800’s. -caused by a fungus, it will defoliate an entire field of potatoes in a few days. -tubers are also infected, and cause the disease to be transmitted over years.

Potato Late Blight, (Phytophthora infestans) Defoliated Plant Infected tuber

5) Dutch Elm Disease, (Ceratocystis ulmi) -a devastating fungal disease of elms, was first discovered in the U.S. in the 1930’s. -a vascular wilt pathogen, it cuts off the supply of water and nutrients to the affected limbs. -vectored by the Elm Bark Beetle -has caused a virtual elimination of Elms

A healthy Elm A diseased Elm

The Elm Bark Beetle A Bark Beetle Feeding On an Elm branch.

A disease resistant Chinese Elm

6) Black Spot on Rose (Diplocarpon rosae) -a fungal disease, it causes black spots to form on leaves. The spots eventually cover the entire leaf, which then falls off. All of the leaves will fall eventually. -defoliation of the plant leaves it open to drought and insect damage. -damaged plants may not live next year.

Black Spot on Rose (Diplocarpon rosae)

7) Fire Blight of Apple and Peach (Erwinia amylovora) -a bacterial disease, spread by rain, as well as by insect vector. -infects through the flowers, in early spring. -causes a distinctive “shepherds crook” on infected limbs. Limbs appear burned and the entire tree will eventually die.

Fire Blight of Apple and Peach (Erwinia amylovora) Apple Limbs With Fire blight Peach Limbs With Fire blight

Control of Plant Diseases There are a variety of methods for the control of plant disease. They fall into a few broad categories. Cultural control: the use of specific techniques or practices to eliminate or mitigate the pathogen. Ex:disinfecting pruning equipment, removal of dead plants and plant debris, spacing of crops to allow drying, crop rotation

2) Biological Control: using plants that are immune to the pathogen, or are at least tolerant. Ex: the planting of Chinese Elms at the UD Campus, instead of American Elms. -Using biopesticides (beneficial bacteria) which outcompete some diseases (mostly Erwinia)

3) Chemical control: the use of some pesticide to kill either the pathogen or its vector. This usually has to be done over the course of a season, not just once. Ex: Spraying fields to kill off the late blight fungus, spraying antibacterial chemicals on fire blighted trees, injecting American Elm trees with insecticide and fungicide to kill the fungus AND the vector.