Phytophthora Crown and Root Rot

Beginner Advanced

Click to print

Click to enlarge.

Beginner

Scientific Name: Phytophthora spp.

Identification

The disease affects red, purple and black raspberries.
High soil moisture and saturated soils create an ideal habitat for growth and dispersal of Phytophthora.
The disease occurs in low-lying, heavy soils, with poor or slow drainage, after heavy precipitation or irrigation.
Plants affected by this fungus appear to be suffering from “wet feet”. Sometimes. entire groups of plants show symptoms simultaneously.
The disease causes a reduction in plant density and new primocane growth in the spring.
Leaves become chlorotic or yellowed with scorching on the outer edges and between veins.
To identify phytophthora crown and root rot in the field, dig up an affected, but not dead, plant and scrape the epidermis of the crown and main root.
Infected tissue will be brick –red with a distinct line between the rotting reddish-brown and healthy white tissue in affected plants.
Confirm this diagnosis at a diagnostic clinic or lab, by microscopic identification of the fungal structures.

Often Confused With
Winter Injury
Raspberry Crown Borer
Verticillium Wilt

Period of Activity
Infections can take place whenever soil temperatures are over 10°C and soil moisture conditions are high (saturated soil). However, phytophthora infections occur most frequently in the spring and fall. If environmental conditions are favourable, several sporulation events can take place during the growing period which results in the rapid spread of this disease.

Scouting Notes
The disease frequently develops on plants grown on heavy, compacted or poorly drained soil, at the lower end of a field or dips within the field. Look for wilted, weak, stunted or collapsed plants, and areas of low cane density and sparse new primocane growth. Confirm diagnosis by digging up plants and scraping away the epidermis of the diseased crown and roots. Phytophthora infections cause a reddish discoloration just beneath the bark of the infected crown. Certain varieties are very susceptible to phytophthora crown rot and are much more likely to develop the disease than tolerant varieties.

Table 1. Field Resistance of Red Raspberry Varieties to Phytophthora Root Rot

Partially Resistant	Susceptible	Very susceptible
Latham¹ Killarney¹ Boyne¹ Prelude² Anne² Nova² Caroline² NY258²	Heritage¹ Taylor¹ Newburgh¹ Royalty¹ Josephine²	Titan¹ Canby¹ Ruby¹ Lauren² Encore² Polana² NY253²

Source:
¹ W. Wilcox and M. Pritts
² Courtney Weber and Jeremy Pattison 2003, Cornell University

Thresholds
None established.

Advanced

Scientific Name: Phytophthora spp.

Identification
Phytophthora crown and root rot is caused by one or more species of Phytophthora. These include Phytophthora megasperma, P.cryptogea, P.citriocola, P. cactorum and a few other, unidentified species. The disease affects red, purple and black raspberries.

High soil moisture and saturated soils create an ideal habitat for growth and dispersal of Phytophthora. The disease occurs in low-lying, heavy soils, with poor or slow drainage, after heavy precipitation or irrigation. Plants affected by this fungus appear to be suffering from “wet feet”. It is common for an entire group of plants, especially those located in low-lying terrain, to be affected simultaneously. Over time, the disease spreads outward and even uphill from the initial site of infection. The disease causes a reduction in plant density and new primocane growth in the spring. Infected plants produce weak lateral shoots. Leaves become chlorotic or yellowed with scorching on the outer edges and between veins. In severe cases, leaves wilt and die and the whole plant collapses before harvest.

To identify phytophthora crown and root rot in the field, dig up an affected, but not dead, plant and scrape the epidermis of the crown and main root. If the exposed tissue just beneath the epidermis is reddish-brown, the plant may be infected with Phytophthora. There is a distinct line between the rotting reddish-brown and healthy white tissue in affected plants. Confirm this diagnosis at a diagnostic clinic or lab, by microscopic identification of the fungal structures.

Often Confused With

Winter Injury
Plants affected by winter injury have similar above-ground symptoms: cane die-back and small, wilted leaves. However, plants with winter injury have abundant and healthy primocane growth, while primocanes are weak and sparse when infected by Phytophthora.

Raspberry Crown Borer
Plants affected by crown borers display wilting, bent shoot tips or complete plant collapse similar to that observed in plants infected with phytophthora. To distinguish the two problems, dig up and examine the roots and crowns of affected plants. Look for tunnelling frass and crown borer larvae, or scrape the bark and look for reddish discoloration which indicates phytophthora.

Verticillium Wilt
Both diseases may result in stunted, weak plants, however, in the case of phytophthora root and crown rot the crown and main root exhibit a reddish-brown discolouration beneath the epidermis.

Biology
Phytophthora is a common soil-born pathogen, with many sources of infestation. It may be introduced to a field on infected new plant stock, contaminated irrigation water or surface run-off from an infested field. Dormant resting spores, or oospores, can survive in the soil for several years, even in the absence of the raspberry host. In this stage, the pathogen is insensitive to environmental conditions and fungicides. (Phytophthora life cycle)

The fungus most often exists as mycelium in infected plant tissue or as dormant oospores in soil. During the spring when soils are cool and have abundant free water, the fungus produces motile zoospores. These zoospores swim through saturated soil pores and are chemically attracted to the tips of raspberry roots. They penetrate the tips of young roots and the fungus grows into the plant. During the summer, when soil becomes warm and dry, the fungus forms dormant oospores in the soil or dead root residues. Infected plants that have not totally collapsed grow new healthy roots. (Phytophthora life cycle)

During the fall, when the soil becomes cool and moist again, oospores germinate and zoospores are released into the soil. The fungus overwinters in infected roots and produces more zoospores in the spring. Zoospores may swim to the soil surface where they can be carried long distances in surface run-off. Zoospores are most active in soil between 7°C and 15°C).