Oculimacula spp.

Scientific Name Oculimacula yallundae (Wallwork & Spooner) Crous & W. Gams [teleomorph = sexual form] Helgardia herpotrichoides (Nirenberg) Crous & W. Gams [anamorph = asexual form], (formerly also described as Wheat-type) and Oculimacula acuformis (Boerema, R. Pieters & Hamers) Crous & W. Gams [teleom.] Helgardia acuformis (Nirenberg) Crous & W. Gams [anam.], (formerly also described as Rye-type)
Synonyms Tapesia yallundae var. yallundae Wallwork & Spooner [obs.], Pseudocercosporella herpotrichoides var. herpotrichoides (Fron) Deighton [syn.], Ramulispora herpotrichoides (Fron) Arx [syn.], Cercospora herpotrichoides Fron [obs.] and Tapesia yallundae var. acuformis Boerema, R. Pieters & Hamers [obs.], Pseudocercosporella herpotrichoides var. acuformis Nirenberg [syn.], Ramulispora herpotrichoides var. acuformis (Nirenberg) Boerema, R. Pieters & Hamers [syn.]
Common Names English: Eyespot, strawbreaker; Spanish: Mancha ocular; German: Halmbruch, French: Piétin-verse

Biology

Damage

Eyespot usually becomes visible in early spring as diffuse brown smudgy areas on the lower leaf sheaths. They develop into characteristic ellipsoid eyespot symptoms on the stem base. Lesions become more defined when the pathogen penetrates the stem and may produce an eye-shaped lesion with a 'pupil' of black dots. Severe infections cause weakening of the stem and lodging at the lesion.

In contrast to lodging from wind, which often is unidirectional, diseased plants tend to lodge in all directions (straggling). Under dry conditions without lodging, severely affected shoots tend to ripen prematurely producing shrivelled grain with low kernel weight. Under moist conditions white heads become colonized by sooty molds and turn to a black appearance.

Infection can occur soon after seedling emergence and may kill the plants when attacks are severe. Severe infection of stems is categorized as where half or more of the circumference of the stem is affected by a lesion causing weakening such that lodging could occur and moderate infection is a half or more of the stem circumference affected by a lesion; when lesions affect less than half the outer sheaths infection is rated slight.

Lifecycle

Infection occurs from conidia that are splashed short distances by rain droplets. Sporulation is optimal at 10 °C and infection will occur when the temperature is above 5 °C and during periods of wet weather (optimum 7-11 °C). Depending on environmental conditions the primary colonization can take up to 8 weeks. Fungal growth progresses through the leaf sheaths to the stem where the economic damage is done. The development of Helgardia species is monocyclic. Dry periods after stem erection may cause early dying of the outer, older leaves which is often associated with a loss of contact between successive sheaths and a halt of the disease. Spores for infection are present throughout the growth period, however, temperatures >16 °C inhibit new infections.

Eyespot lesions form as elongated necrotic areas (<3 cm long) on the outer sheath before the mycelium colonizes the stem producing lightly-colored, later darker-colored lesions. The stromatic mycelium is thick-walled, polygonal and darkly pigmented. Infertile mycelium is usually produced until the following spring. Under optimum conditions spores are produced within 2-7 days. Hyaline conidia with a truncate, unthickened hilum are produced on sympodial conidiophores (<20 µm long, 3-3.5 µm wide) in tufts on free mycelium. The two species differ in spore morphology with H. herpotrichoides producing 5-celled, curved conidia (35-80 x 1.5-2.5 µm), and H. acuformis producing 5-7-celled, straight conidia (43-120 x 1.2-2.3 µm). The anamorph is likely to have short-range dispersal (from rain splash), the teleomorph long-range dispersal. The light-gray, small apothecia are produced from dark primordial on straw debris in spring. They produce hyaline 1-celled ascospores which are released actively and dispersed with the wind.

Cause

Being poor competitors in the soil Helgardia spp. survive on cereal stubble; in addition some grass and weed species may also serve as inoculum source.

Occurrence

Additional Crop Information

Oculimacula spp. can infect wheat, rye, triticale, barley, oats, and several grass species.

Agricultural Importance

The economic impact of the eyespot fungi is highest for autumn-sown cultivars. Yield loss results from the direct effect of the disease in restricting uptake of water and nutrients from the soil and from the consequences of lodging. Significant yield losses only occur with severe and moderate infection. Yield losses in wheat may reach 60 %, however, average annual losses are probably 1-5 %. In spring-sown crops, yield loss is generally negligible.

Control

Integrated Crop Management

Due to moist cool conditions at stem base level the risk of infection increases with heavy clay soil types, excessive nitrogen application encouraging lush growth, dense sowing, early sown crops and short rotations. The fungus is able to survive on buried stubble for at least 3 years, particularly when deeply ploughed and with little rotting of straw. Eyespot is favored by crop rotations with more than 50 % of cereals. Breaks of 2 years from a cereal crop are sufficient to reduce disease risk. Problems may arise when infected stubble is returned to the soil surface after being deeply ploughed under. Eyespot resistance is available in modern cultivars; most cultivars have a moderate level of resistance.

Chemical Control

Eyespot control is most efficient when fungicides are applied at GS 30-32. The action threshold for economic control is 20-30 % of tillers bearing penetrating lesions between first and second node development stage.

Fungicides offer the best protection against eyespot. Efficacy of control varies from 40-80 %. Benzimidazoles, azoles, (DMIs), anilino-pyrimidines and benzophenones provide effective control. However, widespread resistance occurred in the 1980s in Europe against benzimidazoles and in the 1990s locally also against prochloraz (a DMI fungicide). Beside benzimidazoles, DMI fungicides are widely used for eyespot control.

The imidazole compound prochloraz is effective against both eyesport species whereas most triazoles such as tebuconazole show only efficacy against one of the two pathogens, O. yallundae. An exception is the new Bayer CropScience fungicide prothioconazole, showing good control of both eyespot species and no cross resistance to prochloraz.

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