by Antonia Tetteh, Melisa Crane, Tammy L. Ellington and Todd C. Wehner
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609
Watermelon (Citrullus lanatus) is a member of the Cucurbitaceae family. Its center of origin is southern and central Africa, where four species of Citrullus have been identified. These species are C. lanatus, C. colocynthis, C. rehmii , and C. ecirhosus. Commercial cultivars are classified as C. lanatus, var. lanatus, while wild accessions are classified as C. citroides. Plant diseases caused by fungi are a major constraint to the production and marketing of crops. Powdery mildew is an agriculturally significant disease on a wide range of crop species, and has recently been a problem on watermelon in the south and southeast regions of the U.S.
History of powdery mildew
In the past, powdery mildew disease has been a devastating disease of many cucurbits. However, watermelon (Citrullus lanatus) was considered to be resistant. In recent years, outbreaks of powdery mildew disease in watermelons have been reported at least in the southeastern region of the United States – Texas, Georgia, Florida, Arizona, North Carolina and California.
The powdery mildew pathogen
Powdery mildew on cucurbits is caused by two organisms, Podosphaera xanthii (syn. Sphaerotheca fuliginea) and Golovinomyces cichoracearum (syn. Erisyphe cichoracearum). In the past, Erisyphe cichoracearum was considered to be the primary causal organism of powdery mildew. However, in recent years, S. fuliginea is more commonly reported worldwide.
Powdery mildew of watermelon appears in two ways: as white talcum-like powder of mycelia and conidial development on leaves, petioles, and stems, and as chlorotic (yellow) spots on leaves with little mycelia.
Control of powdery mildew on cucurbits is done using fungicides such as benomyl, the triazoles, morpholines, and strobilurins. However, most of these fungicides are phytotoxic to the cucurbits and the fungus may develop resistance to the fungicides. Control of powdery mildew has been difficult because of the occurrence of races in Sphaerotheca. To date, 7 races have been identified in melon, of which race 1 and race 2 appear to be the causal organisms of powdery mildew in watermelon.
There is the need to search for alternative means to control powdery mildew disease in watermelons. Genetic resistance to powdery mildew has been identified in other cucurbits and has been transferred to commercial cultivars beginning in the 1930s. The search for resistance genes against race 1 and race 2 from the USDA plant introduction watermelon germplasm collection is under way. At least 5 of the plant introduction accessions were reported to be resistant to powdery mildew race 1. Our team is working on screening the collection for resistance to race 2.
The objectives of this study were to develop a method for screening watermelon for resistance to powdery mildew race 2, to screen the germplasm collection of 1654 plant introduction accessions and cultivars using 9 replications, to retest the most resistant and most susceptible accessions to verify their performance, and to release the most resistant plants of the most resistant accessions to researchers interested in powdery mildew resistance.
In this study, a wide range of symptoms of powdery mildew race 2 on watermelon were identified. In addition to the white talcum-like powdery mycelia, and chlorotic spots on the leaves, a complete yellowing of the plants, burning of leaf margins which gradually spread inward toward the mid-rib and resulting in total burning of the plant were observed. The disease also manifested softening and sogginess of petioles, followed by rotting and plant death with no powdery colonies.
Preliminary screening results have shown that out of the 1654 germplasm tested, 3% were resistant and 78% were susceptible. The rest were either partially resistant or the seeds were not viable. Future research is planned on the genetics of resistance to powdery mildew race 2 in watermelon.