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Plantwise Technical Factsheet

white tip of leek (Phytophthora porri)

Host plants / species affected
Allium
Allium cepa (onion)
Allium cepa var. aggregatum (shallot)
Allium chinense (spring onion)
Allium fistulosum (Welsh onion)
Allium porrum (leek)
Brassica oleracea var. capitata (cabbage)
Campanula persicifolia (peach-leaved bellflower)
Daucus carota (carrot)
Dianthus caryophyllus (carnation)
Gladiolus hybrids (sword lily)
Hyacinthus orientalis (hyacinth)
Lactuca sativa (lettuce)
Tulipa gesneriana
List of symptoms/signs
Leaves  -  abnormal colours
Leaves  -  abnormal forms
Leaves  -  yellowed or dead
Stems  -  wilt
Vegetative organs  -  soft rot
Symptoms
On leek the first observable symptom is a yellowing of the leaf tips; later they become bleached and white and die rapidly, hence the term 'white tip'. This infection results from direct contact between infested soil and leaf tips. Leaves are often distorted and twisted. Infection progresses from the tip to about half-way down the leaf. Leaf spots are initially water-soaked but turn white and crisp. Spots are surrounded by a green, transparent, water-soaked region. Many infections also start in the water basin that is usually present near the leaf axils. Lesions will appear at some distance above this water basin because of leaf growth during the incubation period. Both young and old plants are affected, and severe infection results in rotting off of leaves at the soil level.

On onion, the symptoms are similar. White spots with water-soaked green zones are present all over the leaf. Severely damaged leaves die off (Tichelaar and van Kesteren, 1967). Another disorder of onions and shallots that is sometimes attributed to P. porri is called shanking: the leaves of affected plants yellow and shrivel, and the base or the whole bulb may become soft and water-soaked (Hickman, 1943; Katsura et al., 1969).

In diseased Campanula plants the crown region is grey-brown to chocolate-brown; this discoloration spreads upward into the leaf bases and flowering shoots and downward into the roots. The upper parts of the roots are pale pink to brown and later turn dark brown and disintegrate, freeing the leafy rosette from the roots (Legge, 1951).

On Gladiolus the pathogen causes a wet rot of the bottom leaves; on carnation dark, wet lesions, later becoming grey and dry, appear on the stalks.

Storage rot of carrots and cabbage and stem rot of lettuce are also attributed to P. porri, but some of these isolates may have been misidentified.
Prevention and control
Introduction

On Allium species, especially leek, control of P. porri is usually necessary to prevent crop losses.

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Early Warning Systems and IPM

Smilde et al. (1996b) studied rain-driven epidemics of P. porri on leek and concluded that disease correlates well with rain data. However, disease forecasts will be unreliable as long as rain forecasts are unreliable. Yokoyama (1976) in Japan showed that the occurrence of P. porri on onions is affected by temperature and rainfall during November-March. From a regression equation using the disease level and temperature and rainfall indices, outbreaks could be forecasted with a 75% probability.
Impact
P. porri is a serious disease of winter leek in Europe which leads to lower yields and loss of quality. Epidemics may destroy more than 50% of the crop before January-April, when the winter leek is harvested. Katsura et al. (1969) described P. porri as a serious problem in Japan, especially in Fukui and Kyoto in winter on Allium bakeri [Allium chinense] and other Allium species. Griffin and Jones (1977) reported yield losses of 30-50% in autumn-sown salad onions in the UK due to P. porri. In Japan, P. porri reduced onion weight by 70-80% (Yokoyama, 1976).
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