EPWS 310 - PLANT PATHOLOGY
Lecture 7: Oomycetes - Order Peronsporales
continued
Contents
of Lecture
Reading: pages
409 - 427
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.