EPWS 310 - PLANT PATHOLOGY

Lecture 7: Oomycetes - Order Peronsporales continued

Contents of Lecture

2. Root Rot Diseases

3. Phytophthora Foot and Crown Rots

4. Phytophthora Leaf and Stem/Fruit Blights

Reading: pages 409 - 427

2) ROOT ROT DISEASES

Serious diseases caused by Pythium and Phytophthora

These fungi are necrotrophs and range from root nibblers to outright root rots.

Pythium species tend to be root tip "nibblers" but may still account for serious yield losses.

Some Pythium species cause damping off and mild root rots usually of younger plants. They are bad in nurseries. Many Pythium species cause root defromation and may only be serious in root crops eg carrot and sugar beets (P. violae, P. ultimum and P. aphandermatum).

Pythium  Disease Cycle Figure 11-18

Phytophthora species are very serious root rot in irrigated agriculture - zoospores.

There are several species which cause root rot of commercial crops and orchard trees. The initial above ground symptoms occur long after infection and colonization has progressed. The plants show wilting and general chlorosis and rapid necrosis and death of the whole plant or large parts of the plant.

Some Phytophthora root rots are very aggressive and can kill whole fields in a short time if condition are right.  What are right conditions?

Older trees show decline and dieback rather than death. see next section

Chile root rot.

These fungi are able to infect most parts of the root system (cf. Pythium) and can grow along the root and up to the crown and stem. Control depends on resistance, good water management, rotation and careful chemical control (resistance).

3) PHYTOPHTHORA FOOT AND CROWN ROTS

Many Phytophthora species cause diseases of the lower stem of many woody hosts. such as nut trees, Citrus, avocado and forest trees. This causes decline dieback and death, P. cinnamomi, P. citrophthora (on citrus).

Injection of Phosphorous Acid (Fosetyl Al) and Metalaxyl have been very successful (because not in soil).

4) PHYTOPHTHORA LEAF AND STEM/ FRUIT BLIGHTS

Many Phytophthora species have deciduous sporangia that are dispersed by wind eg P. infestans.

Rain splash and wind spreads sporangia and these diseases can be VERY serious.

See P. infestans.

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