Thursday, March 20, 2008

Late-season vine decline of melons

The seminar disclaimer applies to this post.

This week our departmental seminar was given by none other than the current President of the profession organization for plant pathologists, the American Phytopathological Society (APS), Dr. Raymond D. Martyn. Dr. Martyn is a homegrown graduate, who did his PhD research on biological control of waterhyacinths. He then moved to Texas A&M, where he was a professor for 20 years, and his research focused on soil-borne diseases of melon. He then moved to Purdue University as Head of the Botany and Plant Pathology Department.

Dr. Martyn is no longer involved in active research but this has given him plenty of time to digest the work he did over the years.

During the past 20-25 years a dramatic increase in late-season vine decline (LSVD) of melons has been observed. This involves all members of the cucurbit family of plants, for example watermelon, squash, and muskmelon. LSVD is a generic term used to describe a set of symptoms that include the sudden death of cucurbit foliage about 2-3 weeks before harvest. This period is critical for the accumulation of sugars during the final ripening stages of the melons, so the vine death results in major losses.

There is no single cause for LSVD, nothing that correlates exclusively with vine decline. Sometimes there are pathogens involved, other times not. Vine decline has reportedly been associated with presence of fungi, viruses, bacteria, application of herbicides, and more. The fungus Monosporascus cannonballus, is one of the major fungal pathogens found to be associated with LSVD.

M. cannonballus is a fungus that forms fruiting bodies called perithecia on the roots of the melons. These perithecia have structures in them called asci (singular: ascus). Fungi belonging to the ascomycota typically have asci with 8 ascospores, M. cannonballus is different in that each ascus only has one ascospore, but this spore contains 8 nuclei (it's a called a multinucleate ascospore). Sometimes 16, but mostly 8. Looking at the picture below, it's easy to see where the fungus gets its name from, the perithecium is almost perfectly round and smooth. The ascospores have very thick walls, and are hard to germinate: they are dormant spores. M. cannonballus was first reported in 1970 as a saprophyte (not a pathogen) on canteloupe, in 1983 it was first reported as a plant pathogen in Israel, and in 1989 it was first reported within the US by guess who... Dr. Ray Martyn.

Picture credit

In the greenhouse, M. cannonballus can infect many different plant species. When a fungus can infect many species, it is said to have a "wide host range." However, M. cannonballus is only a problem on watermelon and muskmelon in the field, so for practical purposes M. cannonballus has a narrow host range. Dr. Martyn studied potential biocontrol methods. He looked at ways in which the disease can be managed using organisms. In this case, M. cannonballus changed in culture, and became hypovirulent, meaning they caused a lot less disease than the original fungus. He investigated whether he could decrease the damage caused in the field by adding the hypovirulent fungus, which would "mate" with the disease-causing fungus, resulting in hypovirulent offspring.

Dr. Martyn authored an article for the APS Education website on Monosporascus root rot and late-season vine decline of melons.

A number of other pathogens were studied as possible causes of late-season vine decline, but all turned out to be fairly weak pathogens, that only caused damage when the plant was very young. In LSVD, the plant is mature, and much stronger, and these pathogens were not responsible for the disease symptoms. In addition, Monosporascus is not the cause of LSVD everywhere in the world. It is reported only in areas that are fairly warm.

Since LSVD is a fairly recent syndrome, Dr. Martyn considered the changes in melon production in the past 25 years. These are:
1. Melon production switched from open-pollinated plots to the use of hybrid melon cultures.
2. Instead of direct seeding into the ground, current production techniques almost exclusively use transplants (seedlings are grown in small trays, and transferred into the ground when they are older).
3. Planting used to be in bare ground, and irrigation systems used furrows, currently plastic mulch and drip irrigation are used.
4. Plants are planted much closer together than in the past.

Dr. Martyn proposed during the seminar that these changes in melon culture practices result in compromised and restricted root systems on the plants. When melons are seeded directly into the ground, the initial root that is formed when a seed germinates, the tap root, continues to grow deep into the soil, as deep as 6-8 feet. A large number of secondary and tertiary roots form along the length of the tap root, allowing the plant to extract water and nutrients from deep in the soil. When seed is germinated in little seedling trays, the tap root grows to meet the bottom of the tray and starts growing in a circle. Once the plant is transplanted into soil, the taproot never recovers, and dies off. Secondary roots that are formed after transplanting do not grow deeply into the soil, but remain about 4" or so below the ground level, and grow essentially horizontally. This drastically limits the availability of water and nutrients to the root system, because the roots do not reach deep enough into the soil.

The compromised root systems have a hard time keeping up with the demand for water of the mature plant, especially during the latter stages, when fruits need to accumulate sugars. Minor pathogens and additional stress, for example lack of water, cause rapid death of the plant in the final stages of fruit development, when the plant needs the resources the most.

Dr. Martyn investigated using cone-tainers, which are very long containers to start seedlings, and they allow for much more growth of the tap root before transplanting, and are so long that the tap root does not reach the bottom before transplanting occurs. After the seedling is planted in soil, the tap root can continue to develop.
Picture credit

In conclusion, LSVD correlates with cultural changes, resulting in vulnerable root systems that cannot handle multiple stresses.

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