Progress and challenges in managing Phytophthora fruit rot of Cucurbits

Authors: Kousik, Chandrasekar - Shaker

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/20/2018
Publication Date: 12/4/2018
Citation: Kousik, C.S. 2018. Progress and challenges in managing Phytophthora fruit rot of cucurbits. Proceedings of the 2nd International Soilborne Oomycete conference, Page 13. https://doi.org/35882.

Interpretive Summary:

Technical Abstract: The oomycete plant pathogen Phytophthora capsici is a major factor limiting production of many vegetable crops throughout the USA. Diseases caused by P. capsici are particularly severe in southeastern USA on Cucurbit crops where conditions favorable for disease development are prevalent during the growing season year after year. There are numerous reviews and manuscripts describing the devastation that can be caused by P. capsici on vegetable crops especially peppers and other vegetable crops. In many instances growers have had to abandon fields severely infested with the pathogen. Similarly many of these reviews also describe strategies to manage diseases caused by P. capsici. Most of these strategies rely extensively on application of fungicides. To that end, numerous effective fungicides are available to manage diseases caused by P. capsici. Phytophthora capsici is known to cause a wide array of symptoms on vegetable crops and these include crown and root rots, foliar blights, and fruit rots. Severe fruit rots have been observed on solanaceous and cucurbit crops in the USA. For example P. capsici is known to cause fruit rot in peppers, buckeye rot in tomatoes and fruit rot in watermelon, melon, cucumber, squash and pumpkins. Fruit rot on cucurbit crops are generally difficult to control because of the long duration the fruit are in contact with soil as they develop and the difficulties in getting the fungicides to the belly of the fruit. In recent years numerous fungicides have been found effective for managing fruit rot of watermelon based on studies conducted in the Carolinas and Georgia and on cucumbers in studies conducted in Michigan. Fungicides such as ametoctardin, dimethomorph, ethaboxam, fluopicolide, mandipropamid, oxathiapiprolin, and others have been found to be effective in managing fruit rot of watermelon and other cucurbit crops. Various fungicide rotation schemes have also been developed to prevent development of fungicide resistance and some fungicides are also marketed as mixtures. Many of these fungicides are also classified as reduced risk by EPA. Though numerous fungicides are available to manage cucurbit fruit rots, under severe weather conditions and heavy pathogen pressure, the effectiveness of the fungicides can be significantly compromised and adequate disease control is not realized. An integrated approach to managing diseases caused by P. capsici has been advocated, which includes the use of well drained fields and fungicides. However, in many cases this is not adequate and there is a critical need for developing resistance to fruit rot in cucurbits. Several sources of resistance to fruit rot in watermelon have been identified in plant introduction (PI) collections and highly resistant germplasm lines have been developed and released to the industry for use in breeding programs. Resistance to fruit rot has also been identified in cucumbers in research conducted at Michigan State University. Several sources of resistance to fruit rot in cucumbers have been identified and in most cases the resistance is associated with aging of fruit. That is, young fruit are susceptible but as they age they become resistant. However in watermelon fruit age was not correlated with resistance. Resistant fruit were resistant at all ages and susceptible fruit were susceptible at all ages. In watermelon, the fruit of the resistant lines are all white fleshed and have low sugar content. Efforts are underway to introgress the resistance into cultivated type watermelon. So far a few lines with red flesh and high levels of resistance have been developed. However, the inheritance of resistance is complex and hence a recombinant inbred line (RIL) population is being developed to decipher the genes controlling resistance. In the near future, an integrated management strategy using a combination of host resistance, fungicides