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Plantwise Technical Factsheet

southwestern cotton rust (Puccinia cacabata)

Host plants / species affected
Bouteloua aristidoides (needle grama)
Bouteloua barbata (six-weeks grama)
Bouteloua barbata var. rothrockii (Rothrock grama)
Bouteloua eriopoda (black grama)
Bouteloua hirsuta (hairy grama)
Chloris (fingergrasses)
Chloris ciliata (fringed chloris (USA))
Gossypium (cotton)
Gossypium barbadense (Gallini cotton)
Gossypium herbaceum (short staple cotton)
Gossypium hirsutum (Bourbon cotton)
Gossypium thurberi (Arizona wild cotton)
List of symptoms/signs
Leaves  -  abnormal colours
Leaves  -  fungal growth
Leaves  -  necrotic areas
Stems  -  mould growth on lesion
Whole plant  -  early senescence
Symptoms
The first disease symptom on cotton is the appearance of small, somewhat inconspicuous, pale-green lesions, which subsequently develop into bright yellow spermogonial pustules, usually on the upper leaf surfaces. Spermogonial pustules may also appear on any of the aboveground plant parts. Within about 10 days of spermogonium formation, cup-like aecia erupt through the lower leaf epidermis. Aecia are large and easily observed. They appear as orange-yellow, circular, and slightly raised lesions produced on the lower leaf surfaces, bracts, green bolls and stems of cotton plants. Severe infections may cause defoliation and dwarfing of bolls. Infection of cotton seedlings may cause death. The first symptoms on grama grass are small, pale-brown uredinial lesions on the leaves. Telia are produced on grama grass either in the uredinium or separately as dark-brown to black raised pustules (Percy, 1993; Nyvall, 1999; Olsen and Silvertooth, 2001).
Prevention and control
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:

Biological Control

Greenhouse and field experiments against southwestern cotton rust have been carried out with the fungal parasite Tuberculina persicina, but without satisfactory results (Percy, 1993).
Impact
The aecial stage of P. cacabata appears to have been reported only from the USA (Arizona, New Mexico, Oklahoma and Texas) and Mexico (Mulder and Holliday, 1971b). The telial stage seems more widespread and, in addition, occurs in Argentina, Bahamas, Bolivia, Brazil and Dominican Republic (Mulder and Holliday, 1971b).

Estimates of losses in the USA and Mexico have occasionally been high. For example, in some locations in Arizona in 1930, yields were reduced by 75% (Percy, 1993). In 1959, losses in south-eastern Arizona and the adjacent corner of New Mexico were reported to vary from 50 to 75% (Percy, 1993). In Mexico, in the cotton district around Lucero, Durango, in 1941, rust caused losses of 50% in some fields. In the major production district at Delicias, Chihuahua, a rust epidemic in 1963 resulted in the loss of 100,000 bales (Percy, 1993).

Losses are much higher when plants are attacked by P. cacabata and the leaf spot fungus Alternaria macrospora at the same time. Under these conditions, the untimely loss of foliage leads to premature opening of unripe bolls, reduced yield, poor quality, and problems in mechanical harvesting (Percy, 1993).

Southwestern cotton rust has, over the years, become sporadic in Arizona, as little cotton is now grown in areas where the disease has historically been a problem (Mary Olsen, personal communication).
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