This article summarizes different crops, what common fungal problems they have, and how fungicide should be used in order to mitigate damage and crop loss. This page also covers how specific fungal infections affect crops present in the United States.
Almonds
Alternaria leaf spot Symptoms of Alternaria leaf spot appear as lesions with tan spots on the leaves. The centers of these lesions become black with fungal sporulation. This infection can lead to tree death within 3–4 years of the first serious outbreak. Orchards in high humidity areas result in the largest yield loss, often in excess of 50%. Yield loss tends to rise every year as the tree becomes weaker each year after infection. Three fungicide applications can achieve 60–80% control of leaf spot.
Anthracnose Anthracnose was not seen on California almonds until the early 1990s. By 1996 it was widespread and causing severe yield losses throughout the state. Typical losses in 1996 were 10–15% of the almond crop with severely affected crops incurring losses of 25%. Under wet conditions, orange spore masses are produced and appear as visible droplets. Lesions on mature fruit are rusty orange and gum profusely. Once the diseased fruit die they become mummies that remain on the tree. The pathogen overwinters in these mummies. 80–90% control can be achieved by applying fungicides to protect the crop before rains begin. The California Department of Pesticide Regulation has estimated that without the fungicides to control anthracnose the state's almond production would drop 15–30%.
Brown rot Damage from brown rot occurs several years after the infection strikes. The primary symptom is fruiting spur loss. Brown rot was first discovered on California almonds in the late 19th century and currently affects most almond-producing areas of California. Brown rot can be controlled using fungicides through bloom in order to protect the flower parts from brown rot attacks. Experiments have demonstrated that 44% of twigs were infected with brown rot when crops were left untreated compared to only 4% when the crop was treated.
Crown and root rot Crown and root rot of almonds is caused by at least 14 different Phytophthora species. The risk of root or crown infection is greatest during cool to moderate temperatures with prolonged or frequent soil saturation. A tree infected with Phytophthora can either undergo a period of slow decline that may last years or it can suddenly collapse and die in spring with the advent of warm weather. Eventually, leaves drop, terminal shoots die back, and death of the tree follows. Once in the root or crown the infection may extend into the crown, trunk, or branches. Currently, crown and root rot are a problem affecting 20% of California's almond orchards with potential yield losses of 50%.
Green fruit rot Green fruit rot can be found throughout virtually all almond-producing regions of California. Green rot is typically controlled by fungicides applied to control other fungal diseases that occur during blooming. It is only when cooler temperatures and heavy moisture is present that almond growers are recommended to make fungicide applications specifically for the disease. When left untreated, green fruit rot can cause up to a 10% yield loss.
Leaf blight Leaf blight of almonds was first discovered in 1950, and by 1983 it had spread throughout the Sacramento Valley. Almond leaf blight is characterized by the death of leaves throughout the summer. The fungus interferes with water conduction in the leaf. In fall and winter, dormant buds are killed by an extension of the lesion and in the spring, flowers are killed. Repeated attacks of leaf blight, though rarely killing more than 20% of the leaves, ultimately reduce tree vigor. Experiments have shown that the use of ziram and captan can reduce the incidence of leaf blight by 75–80% in treated trees.
Almond rust Almond rust is a disease of almond trees that is characterized by angular yellow leaf spots on upper leaf surfaces and rusty brown masses of spores on lower leaf surfaces. It was first discovered in the upper Sacramento Valley area of California but now has subsequently spread throughout both the Sacramento and San Joaquin Valley orchards. It typically emerges in the summer and fall seasons and has been shown to cause rapid and extensive defoliation of trees. Treatments of maneb and sulfur in the early season months (typically spring and summer) have been shown to significantly reduce incidence and severity of rust outbreaks.
Scab Almond scab was first documented in almond orchards in the 1950s. However it did not become a major disease until sprinkler irrigation became popular in the 1980s. By the early 1990s almond scab could be found throughout California almond orchards. Scab infects leaves, fruit, and twigs in almonds causing dark spots to form. Scab lesions look greasy and oily. The major concern with almond scab is partial or complete defoliation of the tree. As with leaf blight use of captan and ziram has been shown to reduce total incidence of almond scab infection.
Shot hole Shot hole affects almonds by disrupting both the leaves and the fruit of the plant. It can result in premature nut ripening and plant defoliation. In some occasions, especially when rain persists in the springtime, complete defoliation and tree weakening can occur. Estimates have shown that approximately 80% of California's almond acres are infected with this fungus. A four-year research project by the Almond Board of California determined that production losses from shot hole could range from 50 to 75%. Captan in one to three applications has been used as the primary fungicide to combat this disease. It has been shown to reduce lesions per fruit by 96%.
Apples
Spraying for control of fungal diseases of apples started in the U.S. some time between 1880 and 1905.
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