Introduction and significance
White tip is one of the most important foliar diseases on leek in western Europe. It has become more important in the UK with increasing leek production and loss of effective fungicides. The disease has been mainly reported from Europe, Canada, and Japan. Phyto-
phthora porri affects various alliums, including bulb
and green onion, and garlic. In Japan, losses of 70% or more can occur in onion crops.
Symptoms and diagnostic features
Early symptoms on leek leaves consist of irregularly shaped, water-soaked lesions. Lesions enlarge to several DISEASES OFVEGETABLECROPS
F
UNGAL
D
ISEASES
27 Severe symptoms of white tip on leeks.
71 centimeters long, appear as elliptical blotches, and are
usually most prevalent towards the tip of the leaf (27,
28). Older lesions develop a bleached white center with
a water-soaked margin when the disease is active (29). In dry conditions, the water-soaked margin may not be evident. Lesions are readily colonized by secondary fungi and sooty molds, making diagnosis difficult in the field because of symptom similarities with other foliar problems such as Cladosporium leaf blotch, purple blotch, or frost damage. Badly affected leaves rot and plants may be stunted or even killed. The disease spreads rapidly in cool, wet weather. On onion and garlic, this pathogen causes water-soaked leaf blight and root rot symptoms.
Causal agent
The cause of white tip is the oomycete organism
Phytophthora porri. The pathogen produces sporangia
that are usually non-papillate and measure 37–75 x 31–48 μm. Sporangia germinate directly to produce a germ tube or release zoospores that measure 10–12 μm. This pathogen can be observed in the lab by floating small pieces of infected leaf tissue, taken from the leading edge of lesions, in water for a few days. Microscopic examination of the leaf pieces will reveal the sporangia. Note that sporangia form sparsely or not at all when cultured on solid agar media. Oospores are also produced and measure 19–36 μm.
Earlier reports indicated that P. porri caused white tip of alliums and was also able to infect brassicas. However, researchers now know that P. porri is host specific to allium hosts and does not infect brassicas. P.
porri tends to have paragynous (rather than amphigy-
nous) antheridia, produces oogonia more consistently, and has fluffy aerial mycelium on V8 agar. The brassica pathogen has been given the name P. brassicae, produces large numbers of sporangia on solid agar medium, has a non-fluffy (appressed) appearance on agar media, and infects only brassicas.
Disease cycle
The disease is soilborne and infection can occur from sporangia or oospores when infested soil is splashed onto the leaves, or when leaves are in direct contact with the soil. In the autumn, severe infection of onion seedlings has occurred when heavy rains flatten the seedlings onto the ground. Mycelial growth can occur between 0–25 ºC, with an optimum of 15–20 ºC.
Control
Rotate onion and leek crops with at least a 3-year break with non-host crops. Use fungicides as appropriate, and apply products having different modes of action to dis- courage development of resistant strains. Avoid planting into wet or poorly draining sites. Prepare soils so that crops are planted into well-draining beds.
References
Griffin, M. J. and Jones, O. W. 1977. Phytophthora porri on autumn-sown salad onions. Plant Pathology 26:149–150. Man in t’Veld, W. A., de Cock, A. W. A. M., Ilieva, E., and
Lévesque, C. A. 2002. Gene flow analysis of Phytophthora
porri reveals a new species: Phytophthora brassicae sp. nov. European Journal of Plant Pathology 108:51–62.
Smilde, W. D., van Nees, M., and Frinking, H. D. 1996. Effects of temperature on Phytophthora porri in vitro, in planta and in soil. European Journal of Plant Pathology 102:687–695.
ALLIACEAE
F
UNGAL
D
ISEASES
28 Close-up of white tip disease of leek.
28
29 Leek leaf lesions of white tip disease showing water-
soaked margins.
72
Puccinia allii
RUST
Introduction and significance
Rust diseases of allium crops occur wherever these crops are grown and are important production factors that cause significant crop damage to onion, leek, garlic, and chives. However, the rust pathogen is comprised of genetically distinct sub-groups; therefore, rust in one part of the world may or may not be the same pathogen as rust elsewhere. For example, rust is the most important and widespread disease of leek (30) in western Europe. Leek crops in California, though, are free from rust concerns while neighboring garlic plantings have been devastated by rust. Examination of the California garlic rust isolates demonstrated that these fungi are different morphologically and genetical- ly from garlic rust isolates in the Middle East region. Hence Puccinia allii is a complex pathogen.
Symptoms and diagnostic features
Initial symptoms consist of small, 1–2 mm in diameter, leaf flecks and spots that are irregularly shaped and white or light tan in color. These lesions increase to 3–5 mm spots that develop the typical, bright orange pustules (31, 32) of rust diseases. Pustules erupt through the leaf surface between the veins, develop on both upper and lower leaf surfaces, and release copious amounts of dusty orange spores. There may be chlorotic halos around the pustules. Rust usually first occurs on the older foliage and subsequently spreads to newer leaves. Severely infected leaves can be almost covered with pustules, turn chlorotic, then finally become tan, dry, and dead (33). Reduced plant size and
yield loss accompany severe rust. In garlic, rust causes severe stunting of plants and bulbs (34). At harvest the outer garlic bulb sheaths can be split and weak, resulting in shattering of the bulb and overall poor quality. Late in the infection cycle the teliospore phase can also be seen in darker brown pustules on diseased foliage (35).
Causal agent
Rust is caused by the fungus P. allii, which is an autoe- cious macrocyclic (full-cycled) rust. However, isolates with shortened life cycles, which lack pycnia and aecia stages, are common and referred to as having hemi- form cycles. The prominent orange pustules are the uredinia stage that produces roughened, one-celled ure- diniospores measuring 23–29 x 20–24 μm. The dark brown, two-celled teliospore is produced later and measures 28–45 x 20–26 μm. In the UK, teliospores are uncommon on leek but readily occur on chives. In California, teliospores are very common on garlic and onion. Pycnia and aecidia stages occur on some hosts such as chives and A. fistulosum.
Because of variation in rust isolate host ranges and morphologies, over the years the allium rust pathogen has been classified into various genera and species, including the following: P. allii, P. porri, P. mixta, P.
blasdalei, Uromyces ambiguous, or U. duris. Currently,
it appears justified to group all garlic, chives, and leek isolates under P. allii, and to consider P. allii as a species DISEASES OFVEGETABLECROPS
F
UNGAL
D
ISEASES
31 Leaf pustules of rust on onion.
31
30 Rust uredinia on leek leaf.
73
complex. Within this P. allii complex, physiological spe- cialization does occur as leek isolates from Europe do not infect onion or chives, and the California garlic isolates do not infect leek.
Disease cycle
In Europe, overwintered crops are a significant source of rust inoculum for spring planted alliums. In the UK, rust is normally active from July onwards and is only inhibited by cold periods in winter. Weedy alliums can
also be hosts and provide inoculum for commercial crops. Urediniospores are the primary inoculum and are spread long distances via winds. Optimum condi- tions for infection are 15º C and 100% relative humidity for 4 hours. The pathogen is active between temperatures of 10–24º C. Rust infection is favored by high nitrogen applications and low potash levels. The fungus does not survive in the soil. For the devastating garlic rust outbreaks in California in the late 1990s, a source of primary inoculum was not identified.
ALLIACEAE
F
UNGAL
D
ISEASES
32 Leaf pustules of garlic rust.
32
33 Field symptoms of rust on garlic.
33
34 Stunted garlic bulbs from severe rust. Healthy bulbs
are on the right.
34
35 Close-up of
garlic leaf with uredinia and telia of rust.
74
Control
Crop rotation to nonhosts may help reduce, but not eliminate, rust pressure. Control allium weeds and plow under infected crop residues to help reduce inoculum. Avoid planting consecutive host crops; for example, in some regions in Europe a single leek crop planted in the spring can often be grown with no or little rust. Apply fungicides, such as morpholine or triazole products, where rust occurs. Frequent applications are required to maintain good control in long season crops. For leek, plant resistant cultivars, which are widely used in Europe.
References
Anikster, Y., Szabo, L. J., Eilam, T., Manisterski, J., Koike, S. T., and Bushnell, W. R. 2004. Morphology, life cycle biology, and DNA sequence analysis of rust fungi on garlic and chives from California. Phytopathology 94:569–577.
Clarkson, J. and Kennedy, R. 1997. Quantifying the effect of reduced doses of propiconazole (Tilt) and initial disease incidence on leek rust development. Plant Pathology 46:952–963.
Harrison, J. M. 1987. Observations on the occurrence of telia of
Puccinia porri on leeks in the UK. Plant Pathology
36:114–115.
Jennings, D. M., Ford-Lloyd, B. V., and Butler, G.M. 1990. Effect of leaf age, leaf position and leaf segment on infection of leek by leek rust. Plant Pathology 39:591–597.
Jennings, D. M., Ford-Lloyd, B. V., and Butler, G.M. 1990. Rust infections of some Allium species: an assessment of germplasm for utilizable rust resistance. Euphytica 49:99–109.
Koike, S. T. and Smith, R. F. 2001. First report of rust caused by
Puccinia allii on wild garlic in California. Plant Disease
85:1290.
Koike, S. T., Smith, R. F., Davis, R. M., Nunez, J. J., and Voss, R. E. 2001. Characterization and control of garlic rust in California. Plant Disease 85:585–591.
Niks, R. E. and Butler, G. M. 1993. Evaluation of morphology of infection structures in distinguishing between different Allium rust fungi. Netherlands Journal of Plant Pathology 99, Supplement 3:139–149.
Smith, B. M., Crowther, T. C., Clarkson, J. P., and Trueman, L. 2000. Partial resistance to rust (Puccinia allii) in cultivated leek (Allium ameloprasum ssp. porrum): estimation and improvement. Annals of Applied Biology 137:43–51. Uma, N. U. and Taylor, G. S. 1986. Occurrence and morphology
of teliospores of Puccinia allii on leek in England.
Transactions of the British Mycological Society 87:320–323.
Sclerotium cepivorum