Solanum nigrum, a new host for powdery mildew disease of Capsicum annuum in the Madurai district of Tamil Nadu, India
In October–November 2004, a severe outbreak of powdery mildew disease incited by Leveillula taurica was observed on Capsicum annuum (cv. K2) (chilli) and caused yield loss up to 80%. A similar type of symptom was also recorded on Solanum nigrum grown in and around chilli fields. Based on the morphological characteristics and cross infectivity, it has been confirmed that the powdery mildew pathogens from C. annuum and S. nigrum are the same. The pathogen from C. annuum could infect S. nigrum and vice versa. Other cultivated plant species were not infected. This might be due to the probable occurrence of physiological races. This is the first report of this organism causing powdery mildew in S. nigrum in the Madurai district of Tamil Nadu. The ability of L. taurica to infect the native host S. nigrum is particularly important because this host plant might serve as an inoculum reservoir for adjacently cultivated C. annuum.
First Report of Powdery Mildew of Greenhouse Pepper Caused by Leveillula taurica in Canada.
In 1999, powdery mildew on cvs. Oberon and Triple 4 of greenhouse pepper (Capsicum annum L.) and cv. FireFlame of chili pepper was reported simultaneously in two commercial greenhouses at two separate locations, Leamington and Vineland, geographically separated by 290 km, in southern Ontario. Losses of 10 to 15% each in the 2 and 3.1 ha greenhouse pepper operations were noted. The greenhouse pepper industry in Canada consists of 89.4 ha with sales of $43.6 million (Canadian). Lesions appeared as a white, powdery coating on the abaxial leaf surface only, generally on the lower foliage of pepper plants, while diffuse chlorotic spots were present at corresponding locations on the adaxial surface. In chili pepper, this chlorosis was restricted to interveinal tissue, causing the leaves to have a somewhat netted appearance. In some cases, pale yellow spots appeared on the adaxial portion of the affected foliage during later stages of disease development. No cleistothecia were observed. Pyriform and cylindrical, hyaline, single-celled conidia were present in equal numbers. Both conidia had a network of crests and granules containing tiny, thornlike projections on the surface. These projections were also evident on the lower half of conidiophores. Pyriform conidia ranged in length from 61.6 to 84.0 μm (mean = 70.8, SE = 0.7) and in width from 14.0 to 25.8 μm (mean = 21.1, SE = 0.4, n = 50), with a mean length to width ratio of 3.4. Cylindrical conidia ranged in length from 54.9 to 80.1 μm (mean = 66.3, SE = 0.9) and in width from 15.7 to 24.1 μm (mean = 18.6, SE = 0.3, n = 50), with a mean length to width ratio of 3.1. Short conidial chains borne on conidiophores consisted of a pyriform conidium first followed by cylindrical conidium. Stomatal penetration and extensive endophytic mycelial growth in the mesophyll layer were evident in foliar tissue cleared with glacial acetic acid:EtOH (1:2) and stained with lactophenol cotton blue. To confirm pathogenicity, conidia from infected pepper leaves were dusted onto the water-misted abaxial leaf surface of 41-day-old greenhouse pepper cvs. Cubico and Edison, and onto the similarly treated adaxial leaf surface of 58-day-old field pepper cv. Renegade. Plants were enclosed in plastic bags for 24 h on a bench. Control plants were treated identically but not inoculated. Inoculated plants developed foliar powdery mildew symptoms, including sporulation similar to that of naturally infected plants. Occurrence of the fungus on the abaxial surface of pepper foliage, presence of endophytic mycelium, and the morphological characteristics of the imperfect state confirm the identity of the fungus. This is the first report of Leveillula taurica (Lév.) G. Arnaud on pepper in Canada. Reference: J. Palti. Bot. Rev. 54:423, 1988.
Studies on the specialization of Sphaerotheca fuliginea Schlecht. (Poll.) and other powdery mildews
The ability of conidia of Sphaerotheca fuliginea from one host to infect other hosts belonging to different genera or families has been demonstrated under Shambat conditions. Conidia from Cucurbita pepo or Cucumis sativus were able to cause infection of Hibiscus esculentus and, similarly, cross-inoculations from H. esculentus to C. pepo or C. sativus were all successful. With Leveillula taurica , it was found that the conidia from H. esculentus were able to infect Faba bona, Euphorbia heterophylla or Lycopersicon esculentum . Similarly, inoculations in the reverse direction on to H. esculentus were also successful. Furthermore, conidia from Corchorus olitorius caused infection of H. esculentus but not of F. bona . On the other hand, negative results were obtained with Erysiphe umbelliferarum (= ? E. polygoni ) from Daucus carota or F. bona . Conidia from either species did not cause an infection on a different host species. These series of tests using S. fuliginea or L. taurica yielded positive results when the inoculations were made during the cooler months of December–February. However, when similar inoculations, even between members of the same species, were repeated in March–May, little or no infection occurred in the majority of tests. When there was an infection the incubation period was much longer and the spread was considerably slower. From field observations both mildews are prevalent in the winter, but rare in the summer (April–July). It is believed, therefore, that if there is any appreciable degree of specialization in both species, it is not an inherent character but, like pathogenicity itself, is probably directly affected by the temperature.