DOWNY MILDEW OF CUCURBITS

DOWNY MILDEW OF CUCURBITS AND THEIR MANAGEMENT

1. CC DOWNY MILDEW OF CUCURBITS COLLEGE OF AGRICULTURAL TECHNOLOGY (Affiliated to Tamil Nadu Agricultural University, Coimbatore-3) Kullapuram (Po), ViaVaigai Dam, Theni-625 562 Course teacher: Student: Dr. Parthasarathy . S Miss. C. Pon Alagammai. Asst.Professor (Plant Pathology) 2015021095

2. © https:#www.google.com

3. NAME OF THE DISEASE : Downy mildew of cucurbits. CASUAL ORGANISM : Pseudoperonospora cubensis (Berkeley & Curtis) Rostovtsev.

4. HISTORY: Pseudoperonospora cubensis (Berkeley & Curtis) Rostovtsev is the causal agent of downy mildew of plants in the Cucurbitaceous (gourd family), including cucumbers, melons, squashes, gourds and watermelons. Berkeley and Curtis first reported the disease in Cuba in 1868.

5. DISTRIBUTION : Downy mildew of cucurbits can be found in temperate areas, such as the Americas, Europe, Japan, Australia and South Africa, tropical regions and some semi-arid regions, such as the Middle East. The disease affects cucurbit crops in the field and those grown in passive or traditional greenhouses.

6. HOSTS: Members of Cucurbitaceae, the gourd family, including, 1.Cucumber ( Cucumis sativus ), 2 . Squash ( Cucurbita spp.), 3.Melon ( Cucumis melo ) and 4.Watermelon ( Citrullus lanatus ).  

7. SIGNIFICANCE : In 1991, 21% of expenses for chemical disease control were allocated to the downy mildew and late blight pathogens, 30% of this was devoted to vegetable crops. The largest percentage of downy mildew control was for grape downy mildew caused by Plasmopara viticola (54%) followed by the cucurbit downy mildew caused by Pseudoperonospora cubensis (10%).

8. These values fluctuate based on weather conditions, acreage and market conditions. Downy mildew is an annual late-season problem for squash and pumpkin growers in the eastern United States. Successful breeding of resistant cucumber cultivars in the mid-20 th century provided adequate control of downy mildew without the use of fungicides.

9. The presence of a more virulent strain of the pathogen on cucumber appeared in the eastern United States in 2004 and has resulted in major economic losses. The disease is a major concern for cucumber growers throughout the eastern half of the United States resulting in substantial economic losses.

10. SIGNS: The key to identifying downy mildew is observing the signs (sporangia and sporangiophores) of the pathogen. Sporangia and sporangiophores are most noticeable during humid conditions (e.g., early morning hours before natural humidity dissipates or immediately following rainfall) on the underside of the leaf.

11. SYMPTOMS: Symptoms on cucumber and squash are angular lesions that are limited by the leaf veins. During periods of leaf wetness from dew, irrigation or rainfall, incipient lesions can become conspicuously water-soaked. This is the earliest symptom produced by the disease, but will disappear as moisture dissipates.

12. Severe infection results in leaves that are completely dead and curled up. This symptom has been described as “wildfire” as the leaves appear to be burned. Symptoms on watermelon and cantaloupe are typically irregular shaped lesions on the foliage that turn brown rapidly.

13. Symptoms on watermelon and cantaloupe are not as distinctive as on cucumber and squash and are more easily mistaken for other diseases such as Anthracnose (causal agent: Colletotrichum orbiculare ), Target spot (causal agent: Corynespora cassiicola ), Alternaria leaf spot (causal agent: Alternaria alternata f. sp. cucurbitae ), Gummy stem blight (causal agent : Didymella bryoniae ) .

14. Early lesions are light green in appearance and become chlorotic and finally necrotic as host plant cells die. Infected leaves may experience an upward leaf curl.

15. SYSTEMIC POSITION: Kingdom : Fungi Phylum : Heterokonta Class : Oomycetes Order : Peronosporales Family : Peronosporaceae Genus : Pseudoperonospora Species : Pseudoperonospora cubensis

16. PATHOGEN BIOLOGY: Like other downy mildew organisms, P. cubensis , is a biotroph or obligate parasite, meaning that the organism requires living host tissue to grow. The organism cannot be propagated on artificial media. P. cubensis overwinters on infected cucurbits, either wild or propagated, in areas that do not experience a hard frost.

17. SPORANGIA AND SPORANGIOPHORES: Pseudoperonospora cubensis forms large (20-40 x 14-25 mm in diameter), lemon-shaped sporangia with a conspicuous papilla. Sporangia appear smooth under the compound microscope but with the scanning electron microscope the finely decorated surface of the sporangium is evident .

18. ©https:#www. google.com

19. The sporulating capacity of purely necrotic lesions is low and that of yellow-necrotic lesions. Warming and drying of the atmosphere, typical of early morning hours, causes twisting of the sporangiophores, which may be of importance for the detachment of sporangia.

20. MANAGEMENT: CULTURAL PRACTICES. Downy mildew severity can be decreased by taking actions that encourage airflow and reduce leaf wetness. However, such actions are often insufficient during prolonged, favorable environmental conditions and in the presence of high inoculum levels. Growing cucurbits in environments where humidity levels can be manipulated can help to manage downy mildew.

21. CHEMICAL PRACTICES Efficacious fungicides include fluopicolide , famoxadone + cymoxanil , cyazofamid , zoxamide and propamocarb hydrochloride. Pseudoperonospora cubensis is known to develop resistance to fungicides very rapidly.

22. Reduced efficacy of mefenoxam , metalaxyl and the strobilurin fungicides has been reported. As a result, the fungicides listed previously should be applied under strict resistance management strategies that include tank-mixing with protectant fungicides, such as chlorothalonil or mancozeb , and rotating with fungicides of different modes of action.