Integrated Pest Management Guide

DISEASES

Readers are referred to the Compendium of Blueberry and Cranberry Diseases for photographs and for more detailed information on diseases. The list of fungicides registered in 2001 may be found in the 2001 Canada Cranberry Pesticide Chart and in the Berry Production Guide (BCMAF).

CRANBERRY DISEASES THAT MAY OCCUR IN BRITISH COLUMBIA

Most diseases on cranberry plants are caused by fungi. Fungi may cause a root rot, a fruit rot, a dieback of the uprights and runners, or a blight on the leaves. Some fungi may cause more than one of these symptoms. Disease development is favoured by environmental factors and vine stress. While some of the upright and leaf diseases have distinctive symptoms and are fairly easy to diagnose, individual fruit rots are difficult to differentiate. Damage on the plants or on the berries is often the first sign of disease in the beds. In many cases, a definitive diagnosis can only be obtained by culturing the causal organism from an infected part of the plant. Disease-like symptoms such as dieback of the vines may in fact be caused by insects such as root weevils or cranberry girdlers. Fungicide applications and good cultural practices will help to prevent damage or to stop an infection. At the moment, because of the lack of a monitoring system for disease organisms, the common practice in British Columbia is to apply fungicide twice during the season as a preventive measure. The first application is done at pre-bloom prior to "rough neck" stage of upright development. The second fungicide application, recommended particularly to dry-pick beds to prevent fruit rot, is done at 50 - 80% out of bloom.

GOOD CULTURAL PRACTICES TO REDUCE VINE DISEASE AND FRUIT ROT

• Avoid or minimize sources of vine stress such as picking injury
• Optimize vine vigor by sanding, improving drainage, optimizing fertility, avoiding mechanical damage, and managing pests
• Time fungicide applications to obtain continual protection during periods when vines and berries are vulnerable to infection
• Optimize uniformity of fungicide coverage
• Avoid irrigation in the late afternoon and evening
• Remove harvest trash to reduce inoculum for next year and keep trash pile at least 0.4 km (about 1/4 mile) from beds
• Sand vines to bury inoculum
• Protect vines during heat-stress periods
• Avoid excessive vine growth from heavy fertilizer (nitrogen)

UPRIGHT DIEBACK

In the Pacific Northwest, the fungus Phomopsis vaccinii (the asexual stage of Diaporthe vaccinii) is associated with this disease. The same fungus also causes the fruit rot called viscid rot.

Symptoms: In spring, leaves of infected uprights turn yellow, then orange and bronze. Vines then turn brown and die before bloom. Affected uprights may be present with healthy uprights on the same runner. The disease does not affect roots. The disease usually appears on scattered uprights or in patches. Dieback may also occur during the growing season. Hot and dry conditions favour the development of the disease.

Control: Providing moisture and cooling vines by sprinkling water during hot and dry periods should create unfavourable conditions for spread of the disease. Fungicide applied soon after bud break is the most effective control. Good cultural practices will also reduce development and spread of the disease.

ROSE BLOOM

This is a very apparent and well-known disease caused by the fungus Exobasidium oxycocci. Diseased uprights have abnormal branches with fleshy pink leaves that look like miniature roses.

Symptoms: When vine growth resumes in spring, infected axillary buds grow into fleshy, abnormal lateral branches. Rose bloom may be seen scattered throughout the beds from late April until mid-June. The fleshy branches are first pale green, then turn pink as they enlarge and finally powdery white when spores are produced on the surface. The white, powdery branches become dry, dark brown, hard and withered by mid-bloom. Flowers and berries are occasionally affected. Most newly infected buds will not show the abnormal growth until the following spring.

Control: Control is only advised if many rose bloom growths are observed. Control is based on protecting new upright growth during the period of spore production (early May through mid-June). Application of fungicide should begin with the appearance of spores on the surface of the fleshy pink growths. During this period no more than three applications are recommended at 14-day intervals. Effective control will reduce the number of rose bloom growths the following spring.

TWIG BLIGHT

Two fungi, Lophodermium oxycocci and L. hypophyllum, found only on cranberry can cause twig blight. It is not a serious problem in cranberry bogs in B.C.

Symptoms: Infection occurring in the summer will show damage only in the following spring on the newest growth that will turn brown. The fungi kill only the one-year-old wood on uprights and runners. Infected cranberry leaves are dull instead of glossy. Later in the spring, the infected leaves turn a bleached tan and, eventually, a silver-gray colour. Black football-shaped spore-producing bodies form on the lower surface of blighted leaves. By mid-summer, leaves drop to the bog floor. The pathogens do not survive or produce spores on dead leaves in the trash layer.

Control: The infection period varies from year to year and lasts 4 - 6 weeks in the summer from late June to mid-August. Control is based on protecting new growth during the period of spore release in the beds. Fungicide applications are recommended. Cultural practices have little or no impact on this disease.

PROTOVENTURIA LEAF SPOT AND BERRY SPECKLE

Protoventuria leaf spot and berry speckle disease has been reported in British Columbia. It is common only in cranberry beds that have not been treated with fungicides. This disease causes little damage. The speckles on berries make them less attractive for fresh-market sales but does not affect their keeping quality.

Symptoms: At the end of the summer, small distinct red to purple lesions appear on the surface of the cranberry leaves. In the following spring, the lesions are enlarged but less well-defined. Minute black fruiting bodies clustered in the centre of the lesions may be seen in the summer. At that time many of the affected leaves may drop. Speckling appears on berries in the summer as tiny red lesions on green fruit. They increase in size and number during berry development. On ripe fruits, the lesions appear pale yellow, dark red, or black and color is lighter in the centre.

Control: Fungicides applied in the summer to control fruit rot also control this disease.

COTTON BALL (TIP BLIGHT)

The same fungus, Monolinia oxycocci, causes disease at two different stages of development. In the spring, spores of the fungus infect new upright growth causing tip blight. A second type of spore produced on the dead uprights infects flowers, leading to the fruit rot called cotton ball or hard rot. The disease is economically important on cranberry in British Columbia, reducing both yield and fruit quality. It is most common on Pilgrim and Bergman.

Symptoms: The disease originates from overwintering structures (called mummies) of the fungus in the remains of infected berries in the trash layer of the beds. Spores released from germinating mummies infect new growth on cranberry uprights. Infected upright tips bend over like a shepherd's crook, showing grayish-white masses of spores. The leaves turn a tan colour and fall off when dried, leaving little evidence of tip blight. Spores from infected uprights may infect open flowers and cause fruit rot. Young infected berries show no external symptoms of disease. The fungus develops inside the berry. The name "cotton ball" originates from the white cottony mass of fungus inside infected berries. Symptoms become visible externally on berries when they begin to ripen. Infected berries remain a greenish yellow with tan stripes or blotches and are usually firm.

Control: Recent work in Wisconsin showed that the number of protective fungicide applications required depends on the previous year's disease pressure. If disease pressure was low to moderate (fewer than 15 % of berries infected), two fungicide applications are recommended: one at 10 - 20% bloom and again 7 - 10 days later. If disease pressure was high in the previous year (greater than 15% of berries infected), two additional fungicide applications are recommended: one when about half the shoots have started to elongate and the second 7 - 10 days later (McManus 1999).

RED LEAF SPOT

This disease most often appears in young bogs with excessive growth due to high nitrogen.

Symptoms: As the name describes, the main symptom is the appearance of glossy red spots on the upper surface of cranberry leaves. Below the spots on the underside of the leaf, the spores form a dusty tan spot. Red leaf spot alone does not do damage unless the fungus spreads from the leaves to the stems, causing the death of the new upright tips.

Control: This disease is not usually severe enough to cause economical damage. Since there is no registered fungicide for this disease, cultural practices may help in reducing infection. Fungicides applied for control of fruit rot seem to be effective for red leaf spot. Red leaf spot can be minimized by avoiding excessive vine growth in new beds. This disease is often seen on overfertilized, young plantings of Stevens.

PHYTOPHTHORA ROOT AND RUNNER ROT

This disease has not been found in British Columbia on cranberry. It is mentioned here because, in some cranberry growing areas, it is the most important disease of cranberry roots. Several species of Phytophthora can cause the root rot but Phytophthora cinnamomi is the most virulent one. P. cinnamomi is present in Oregon and Washington and is the primary causal agent in Massachusetts and New Jersey. Other species of Phytophthora seem widespread in cranberry growing regions. It is reported to be most severe in low or poorly drained areas within beds.

Symptoms: The most apparent symptom is the absence of vines in small patches in cranberry beds. In the early stages vines appear weak and unthrifty, there are few, small leaves, flower and fruit production are reduced, small fibrous roots are lacking, and vines are easily pulled from the ground. The cranberry girdler can cause symptoms similar to Phytophthora root rot. However, symptoms of Phytophthora appear mostly in the spring, while injury from girdler becomes apparent at the end of the summer. Girdler damage will show characteristic chewing on the roots.

Control: This root disease can be controlled by improving drainage of the soil. Excessive irrigation should be avoided. Low areas in the beds where water accumulates can be sanded. Fertilize plants to stimulate root growth. The fungus is usually introduced into new areas by planting infected vines.

FRUIT ROTS

Various fungi may cause cranberry fruit rots in the beds or after harvest in refrigerated storage. The causal organisms are usually present in the fields throughout most of the growing season. Infection occurs during bloom and/or early fruit set, depending on the fungus. Infected berries change colour or consistency and it is nearly impossible to identify the rot just by looking at the berry. Fungicides are used to control the various fruit rots. Where fruit rot was a problem the previous year, two or three fungicide applications are usually recommended to protect the berries at the early stage of development. Protection is needed before any symptoms appear. Applications during bloom and early berry development are usually most effective.

END ROT

End rot is an important disease occurring primarily in storage. Although also present in the beds as a leaf spot and a twig blight disease in British Columbia. It is not usually economically significant at these phases.

Symptoms: The early symptoms on the berries are a soft, watery rot starting usually at the blossom end of the berry. Later the whole berry becomes soft and elastic and filled with gas produced by the rotting process. Berries may burst from the increasing pressure of the gas. Reddish brown spots develop on the upper surface of the leaves when first infected. Spots then turn tan or gray in the center with a black border as they expand. The surrounding part of the leaf may turn red. Minor twig blight can appear if the fungus invades young twigs. The fungus overwinters in the old bark on the vines and in dead leaves and rotted fruit in the trash layer.

Control: Since end rot is mainly a storage rot, control is usually recommended only if berries are to be used for fresh market. Fungicides applied to control other fruit rots also control end rot. Berries may be predisposed to end rot if too much nitrogen fertilizer is applied, if bruised during harvest, or if subjected to high humidity and poor aeration during storage.

VISCID ROT

Viscid rot is mainly a storage disease but may cause also rot in the field.

Symptoms: Infected fruits are soft, off-coloured and may be slightly mottled. This disease cannot be identified without isolation of the causal organism. The same fungus causes upright dieback of cranberry. The fungus may be present in the beds of healthy looking uprights, flowers and berries early in the season.

Control: Fungicide applications during early bloom, late bloom and berry development are recommended if disease pressure was high the previous year.

COTTON BALL (HARD ROT) see COTTON BALL (TIP BLIGHT)

BLACK ROT

Black rot is a disease that develops on berries after harvest in storage. The causal organisms present in the trash layer of cranberry beds at harvest infect the berries when beds are flooded for harvest. Disease incidence is directly proportional to the time berries remain in floodwater. During storage, wounded berries may also be infected by the fungi that tolerate temperatures of 2-4ºC. Black rot also occurs in dry-harvested fruit.

Symptoms: Infected stored berries turn black or are mottled grey-brown. The berries progress from dry and firm to dry and shrivelled.

Control: Fungicide applications have not been effective in controlling this disease. Reducing the time that fruits remain in the water during harvest and accelerating the drying of fruit after harvest may reduce disease development. Storing fruit at low temperatures near 0º C reduces the extent of black rot.

BITTER ROT

This fruit rot occurs sporadically. The fungus overwinters on infected parts, dead leaves and rotten berries from the trash layer. Fruit are infected during the early stages of development. Most of the rot occurs before harvest and early in the storage of fresh fruit.

Symptoms: This rot does not have definitive symptoms. It is necessary to culture the fungus from rotted berries to identify the causal organism.

Control: Fungicides applied to control other fruit rots also control bitter rot. Cultural practices such as good water management and appropriate use of fertilizer may reduce the spread of bitter rot.

Other fruit rots, such as blotch rot, ripe rot and Pestalotia rot have also been isolated from rotten berries in British Columbia. Cultural practices used to reduce other fruit rots will also reduce these rots.

References cited in this chapter:

McManus, P. S. 1999. Cottonball disease of cranberry. University of Wisconsin-Extension. Publication A3194.

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