Plant Disease of the Week No. 23c (Rust of lemongrass)

Issue No. 23

June 17, 2008

Plant Disease of the Week

In This Issue

Rust of lemongrass

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Plant Disease of the Week is a series of online bulletins about plant diseases that affect farms, nurseries, gardens, landscapes and forests in Hawaii. You are welcome to forward this message to anyone who might benefit from the information by using the link at the bottom of the page. You may unsubscribe at any time by using the "SafeUnsubscribe" link at the bottom of this page. If you wish to print the document, please exit your email browser and go to the archive and print it from there.

Rust of lemongrass

Puccinia nakanishikii

West Indian lemongrass (Cymbopogon citratus) is a commonly grown perennial grass in Hawaii that is preferred by homeowners as an ornamental plant and for its lemon-scented oil. The oil is used as an ingredient in Asian cuisine in Hawaii and also acts as a central nervous system depressant.

Lemongrass plants in Hawaii often have abnormally brown and dying leaves. Depending on climatic conditions, lemongrass can become severely infected with a rust disease caused by Puccinia nakanishikii that is responsible for leaf necrosis. Heavily infected leaf tissues become discolored and necrotic in streaked patterns that correspond to leaf veins.

The rust disease can damage plants severely enough that significant economic damage would likely result if lemongrass was cultivated as a large scale, commercial crop in Hawaii. Backyard plants can be very damaged by the disease in higher rainfall areas or seasons.

In the bulletin we discuss the pathogen and the symptoms of lemongrass rust and what can be done by growers to manage this common and damaging rust disease.

Disease name: lemongrass rust

Host (common name): West Indian lemongrass, Lanpine (Hawaiian), Lukini (Hawaiian).

Host (scientific name): Cymbopogon citratus (DC.) Stapf., native to southern India and Ceylon.

Other reported hosts of the pathogen: Cymbopogon nardus (Sri Lanka) and perhaps some other species of Cymbopogon.

Photograph (right): Symptoms of rust on West Indian lemongrass (Cymbopogon citratus) in Hawaii, caused by the plant-pathogenic fungus, Puccinia nakanishikii (Photo - S. Nelson (UH-CTAHR)).

Cymbopogon citratus: Indian lemongrass is a densely-tufted perennial grass. The hollow, cane-like, clump-forming stems support linear, rough-margined, strongly scented leaves that are pale blue-green in color, about 36 in (90 cm) long. The plant is grown in Hawaii for its fragrance and is often used in Asian cooking for flavoring food. Leaves are dried and used for tea and sometimes added to bathwater for medicinal effect.

The plant genus Cymbopogon: comprises at least 40 species of evergreen, tufted perennial grasses known for their essential oils that have cosmetic, culinary and medicinal uses. They grow in warm-temperate, subtropical and tropical regions such as savannah grasslands in Africa and Asia. In Java, India and Ceylon the grasses are cultivated widely.

Host family: POACEAE

Pathogen name: Puccinia nakanishikii Dietel

Type of pathogen: fungus

Pathogen taxonomy: Kingdom FUNGI; Phylum BASIDIOMYCOTA; Family PUCCINIASTRACEAE

Pathogen attributes: The rust occurs in both the uredinial and telial states in Hawaii, the former producing lighter brown and the latter producing darker pustules on both upper and lower leaf surfaces. Ellipsoidal urediniospores measure from about 22 to 28 μm by 22 to 25 μm and contained 3 to 4 germ pores in an equatorial configuration.

History of the disease in Hawaii: the first publication to report the disease in Hawaii was in 1985 by Gardner.

Geographic distribution of lemongrass rust: the disease has been reported in Hawaii, California, Thailand, New Zealand and probably at some other locations where lemongrass is cultivated. The disease can occur virtually everywhere lemongrass grows in Hawaii, but is more severe in warm, higher rainfall locations.

Conditions favoring disease development: rainfall, high humidity and warm air temperatures; wind disseminates spores for short and long distances among lemongrass plants.

Disease cycle: There is very little published research about the disease cycle of lemongrass rust. Spores (mainly urediniospores) are dispersed by wind or splashing rain or irrigation water and land on wet or moist lemongrass leaves or during periods of high relative humidity and cause infections which eventually cause erumpent lesions that release more spores for further infections. Spores may survive on infected or fallen lemongrass leaves.

Sites of infection: leaves

Disease symptoms: initial symptoms of light yellow, tiny spots that develop into brown-colored spots and elongated, stripe-like, dark brown lesions that coincide with leaf veins and develop on both sides of leaf surfaces. Lesions on abaxial leaf surfaces erupt and develop dark, cinnamon brown uredinial pustules. Lesion development can be substantial, and coalescing lesions can cause significant leaf spots or blight, foliar necrosis and premature death.

Principal negative effects of lemongrass rust upon plants: defoliation (direct effect); poor yields (indirect effect); reduction in oil production. The rust disease is normally not fatal to lemongrass plants even though defoliation may be severe.

Integrated management practices for rust of lemongrass:

Keep plants growing vigorously, use composts, mulches and/or fertilizer to stimulate growth of plants.
Intercrop or polycrop lemongrass with non-hosts of the pathogen; avoid planting large numbers of lemongrass plants next to each other.
Do not purchase or distribute rusted plants.
Grow plants under plastic or rainproof cover to protect their leaves from rainfall.
Periodically prune, cut back or thin out diseased lemongrass plants so that disease-free re-growth can occur for a period of time, and destroy the diseased material that was cut off (do not place it around pruned lemongrass plants as mulch).
Keep weeds under control around lemongrass plants to reduce relative humidity in the lemongrass plant canopy.
Plant lemongrass in well-drained soils in a relatively dry or well-ventilated area to minimize duration of leaf wetness after rainfall.
Minimize overhead irrigation of lemongrass plants; they grow well in dry areas.

Fungicides registered in Hawaii for lemongrass rust:There is only one fungicide product registered for use on lemongrass rust in Hawaii, Trilogy (Table 1.). There is no published research in Hawaii that has evaluated this product for controlling rust, although there is probably some control of the disease.

Table 1. Fungicides currently registered in Hawaii for application to lemongrass. The only product that lists rust on the pesticide label is Trilogy.

Product name and EPA Registration number*

Ridomil Gold EC Fungicide, 100-801

Rampart, 34704-924

Rootshield granules, 68539-3

Messenger (Disc.), 69834-2

EBC-151 (Disc.), 69834-2

Messenger Seed Treatment, 69834-2

Trilogy (neem oil), 70051-2

Fungi-Phite, 83472-1

*Ridomil, Rampart, and Fungi-phite are for Oomycete pathogens (e.g., Phytophthora and Pythium) that normally infect roots in wet conditions, they are not for rust pathogens. Messenger and EBC are harpin protein products that supposedly increase plant health and defenses against plant diseases. Rootshield is a biopesticide product that protects against soilborne diseases (e.g., root rots). The information in Table 1 was obtained from the Hawaii Pesticide Information Retrieval System (HPIRS) using the keyword "herb." The data were provided by Mike Kawate (UH-CTAHR).

Biological control of the rust disease of lemongrass: A potential biological control agent, a Darluca mycoparasite species, was often observed in uredinia of diseased lemongrass in coastal counties of California (Koike, 1999). It is unknown if this mycoparasite exists in Hawaii or the extent of the mycoparasitism and whether or not it provides effective disease control.

Edibility of infected plants: Lemongrass plants with the rust disease are safe for humans to use in cooking recipes or as teas after drying leaves or as flavoring for beverages or as additives to cosmetics.

Symptoms of lemongrass rust in Hawaii

Initially, the infections appear as tiny yellow spots that develop into the larger brown spots or stripe-like lesions along or near leaf veins. The brown-colored spots and the elongated, stripe-like, dark brown lesions develop on both lower and upper sides of leaf surfaces. Lesions on the abaxial leaf surfaces erupt and develop dark, cinnamon brown uredinial pustules. There may be small, chlorotic yellow halos around lesions.