In 1868, downy mildew of cucurbits of was first described by Berkeley and Curtis in Cuba. Downey mildew of cucurbits is caused by an organism known as Pseudoperonospora cubensis. This pathogen has the capability to damage all the crops of cucurbit family such as watermelon, cucumber, pumpkin, squash, and melon. According to the research, downy mildew of cucurbits disease had been discovered in many countries like Japan, Europe, America, Australia, South America, and Middle East. Infection of cucurbits corps are not very popular in Asia and Africa. In the past, the outbreak of this disease leads to a significant economic loss in United States of America. For example, United States of America lost 16 million dollars because of an outbreak of the disease on cucumbers in 2004. (Colucci, and Holmes, 2010).
P. cubensis is an organism that cannot survive by themselves. They require living tissue for development. The organism is part of Kingdom "Straminipila" and Phylum "Oomycota". Hot, high humidity level, and moist environment are considered favorable conditions that enhances the activity of the pathogen in damaging the cucurbits plants. On the other hand relatively cold temperature like "hard-frost" atmosphere does not support the growth of the pathogen. Pseudoperonospora cubensis forms into a survival structures known as sporangiophore, sporangia, and zoospores. Survival structures such as sporangiophores and sporangia can be viewed under Compound, and or electron microscope. Sporangia would appear on the tips of the sporangiphore. Oospores are the structure produced by the pathogen through sexual reproduction, but their function on cucurbits plant is not known. (Colucci, and Holmes, 2010)
Pathogen can survive for a longer time in or on the infected parts of the cucurbits under high levels of humidity and moisture. Often, the pathogen spores called as sporangia are spread by air. Once the spore reaches a healthy plant then depending on the level of moisture and humidity, it produces survival structures known as zoospores. Favorable conditions facilitate the penetration of the pathogen in the tissues of the plant through germ tube produced by the zoospores. After the infection, the pathogen spreads by travelling inside as well as outside of the plant cells by growing into "mycelium and "Haustoria" which provide food to the pathogen from the plants. (Colucci and Holmes, 2010).
Even though there is one pathogen causing the disease, the symptoms of the disease could be different depending on the type of the cucurbit plants. For example; the pathogen might cause spots at an angle, chlorotic lesions, tissue death known as necrosis, on an individual cucurbit plant, and one of the symptoms that could be seen on the leaves when they fold upwards whereas on other cucurbits plant such as watermelon and cantaloupe the symptoms will occur as small spots that might turn into brown to black in color during the infection period. Disease is also known as "wildfire" because it tends to spread very quickly similar to fire. Even though only leaves are infected by the pathogen, damaged leaves could increase the possibility of damaging the fruit body and reducing the yield. Since the leaves are damaged, the fruits are more exposed to direct sun rays which causes sunscald in fruits. (Colucci, and Holmes, 2007)
(DOI: 10.1094/PHI-I-2010-0825-01)
Figure : Burned and dried leaves of the cucurbit plant
Following figures illustrates the symptoms of the disease. Retrieved from http://www.cals.ncsu.edu/plantpath/extension/fact_sheets/Cucurbits_-_Downy_Mildew.htm
Figure 2: "chlorotic, angular lesion typical of downy mildew on up surface of cucumber leaves. The lesions appear angular because they are bound by leaf veins." (Colucci &Holmes,2007)
Figure 3: infected watermelon leaves Figure 4: infected cantaloupe leaves
The downy mildew of cucurbits can be identified by the presence of the pathogenic structures known as sporangia and sporangiophores on the backside of the leaves during wet and humid conditions. While looking for the infection on the plant, the backside of the leaves specifically show, a moldy and dewy growth on the veins which indicate the presence of the infection. The molds can range in color which included gray-brown to deep purple. It would be determined that the infection is old based upon the density of the mold. The darker the mold, the older the infection. (Colucci, and Holmes, 2010).
The following figures illustrate the signs (presence of pathogenic structures) of the disease. DOI: 10.1094/PHI-I-2010-0825-01. Retrieved from: http://www.apsnet.org/edcenter/intropp/ lessons/fungi/Oomycetes/pages/Cucurbits.aspx.
Figure 5 Figure 6
Downy mildew of cucurbits can be controlled by replacing non resistance cucurbits plant with the disease resistance plants (cultivars); and avoiding the plantation in the atmosphere that contributes to the development of the pathogen. Current study observed the reaction of the specific cucumber cultivars towards the pathogen on different geographical regions around the world such as North America, Poland, India, and China. The study concludes that cucumber cultivars on these different regions develop different levels of resistance. For example; Cultivars that are resistance in India, and China might not be very resistance in Poland, and North America. (Shetty et al., 2002). Forecasting system of downy mildew for cucurbits was introduced in 1998. The objective of the forecasting system was to initiate the application of fungicides at an accurate time period in order to lower the risk of the disease. Disease can also be managed by applying recommended and effective fungicides. (Colucci and Holmes, 2010). A recent research founded that fungicide known as dimethomorph is considered very beneficial in managing the downy mildew of cucumber. Dimethomorph has highly defensive characteristic that inhibits the growth and development of the pathogen on cucumber. (Wang et al., 2009). Whereas, (Ishii et al., 2001), in their experiments researchers detected that pathogens of downy mildew as well as powdery mildew on cucumbers are resistant to Strobilurin fungicides which includes azoxystrobin and kresoximmethyl. The experiment also explains that the gene mutation could have helped these two pathogens to become resistant to the Fungicides. According to the records, during the time period between 1998 and 1999, Japan was unable to control powdery mildew and downy mildew on cucumber by using azoxystrobin, and Kresoximmethyl fungicides .(Ishii et al,. 2001). According to the research, if plants are grown in an environment where accommodation on favorable humidity, space, and location can be provided then the infection can be avoided all together. Favorable locations include greenhouses that have less humidity making the leaves less wet and less allowing of the infection. (Colucci and Holmes, 2010).
