Watermelon Transplant Diseases

A recent observation of gummy stem blight on a watermelon transplant has reminded me to remind growers to inspect seedlings. Whether one is growing transplants or receiving transplants for delivery, seedlings should be inspected for possible disease problems. If one is uncertain of the cause of the symptoms, an official diagnosis can be obtained by sending the sample to the Plant and Pest Diagnostic Clinic. Below I will describe several common transplant diseases of cantaloupe and watermelon as well as management options. I have had similar articles in the Hotline in the past, however, I will use new photos here.

Gummy stem blight on transplant seedlings may be recognized by the water-soaked area of the stem near the seed leaves. In this article, I will show a leaf with a lesion of gummy stem blight (Figure 1). A closer look (one may need a 10X hand-lens) at any gummy stem blight lesion may reveal dark, raised fungal structures.

Figure 1: A leaf lesion of gummy stem blight on a watermelon transplant.

Figure 1: A leaf lesion of gummy stem blight on a watermelon transplant.

The fungus that causes gummy stem blight (Didymella bryoniae) may survive in crop debris, thus overwintering in the field from year to year. This fungus may also be introduced through seed or transplants. It is also possible for the fungus to survive in greenhouse production facilities.

Fungicides that are likely to be effective against gummy stem blight and labeled for greenhouse use include:

  • Mancozeb products (e.g., Dithane®, Manzate®, Penncozeb®, Roper®) are contact materials.
  • Products with the active ingredient tebuconazole (e.g., Monsoon®, Vibe®, Toledo®)
  • Luna Experience®
  • Switch®
  • Quadris Top®; The Quadris® portion (Group 11) of this product will be more effective against anthracnose; the Top portion (Group 3) will be more effective against gummy stem blight.

Anthracnose of watermelon is another disease that may be observed on transplants. The lesions caused by anthracnose (Colletotrichum orbiculare) are often jagged or sharp in appearance (Figure 2). Stem lesions are less common, but if they occur they may appear water-soaked, light brown and pitted. Such stem lesions will not necessarily appear at the seed leaves. Anthracnose on cantaloupe transplants is less common.

Figure 2: A watermelon transplant with lesions of anthracnose.

Figure 2: A watermelon transplant with lesions of anthracnose.

As described above for gummy stem blight, the fungus that causes anthracnose may survive in crop debris such as in transplant production facilities. This fungus may also be introduced through seed or transplants.

Fungicides that are likely to be effective against anthracnose and labeled for greenhouse use include:

  • Mancozeb products (e.g., Dithane®, Manzate®, Penncozeb®, Roper®) are contact materials.
  • Luna Sensation®
  • Topsin® forumulations
  • Quadris Top® (see above);

Watermelon transplants with Fusarium wilt often appear wilted or the plant tops may have died back (Figure 3). Symptoms that appear under humid greenhouse conditions may be accompanied by white mycelial growth of the causal fungus, Fusarium oxysporum f.sp. niveum. Seedlings with Fusarium wilt symptoms may be clustered in transplant trays.

Figure 3: Fusarium wilt symptoms on watermelon transplant.

Figure 3: Fusarium wilt symptoms on watermelon transplant.

Fusarium wilt may be introduced through seed or transplants. Unfortunately, the fungus that causes Fusarium wilt has long lived spores that may survive for years in soil, equipment or transplant trays.

There are no fungicides that have been proven to be effective against Fusarium wilt of watermelon in the greenhouse.

The symptoms of bacterial fruit blotch (BFB) can be difficult to recognize on foliage. Leaf lesions may have a dark necrosis with water soaked margins (Figure 4). Leaf symptoms of BFB are easily confused with angular leaf spot, a disease that is not often economically important. A laboratory analysis may be required to distinguish these two diseases.

Figure 4: Symptoms of bacterial fruit blotch on a watermelon transplant.

Figure 4: Symptoms of bacterial fruit blotch on a watermelon transplant.

The bacteria that cause BFB do not often survive in crop debris; the disease is more often transmitted through seed. Although symptoms are more often observed on watermelon, cantaloupe transplants may also be affected.

To guard against these diseases in your field, carefully inspect transplants regularly during production or upon delivery. If unsure about symptoms, send them in to the Plant Pest and Diagnostic Laboratory or a similar laboratory for an official diagnosis.

Clean and sanitize transplant production facilities and equipment in-between generations. Purchase transplant trays for each generation of transplants or clean and sanitize trays well. Do not use soilless greenhouse mix that has been opened or come into contact with the ground or unclean equipment.

Purchase vegetable seed that has been tested for the diseases described above. Ask your seed company representative if you are uncertain about what tests have been conducted.

Finally, avoid planting transplants grown from seed lots or greenhouses where any of these diseases has been confirmed. Seedlings that appear healthy may in fact have a disease that has spread from a nearby seedling.

In most years, it will be impossible to avoid at least some of the diseases described above. But, as much as possible, do not plant these diseases with your transplants. In particular, avoid using transplants with Fusarium wilt. Since the Fusarium wilt fungus survives many years in the absence of a host, an introduced fungus may last indefinitely. Plus, watermelon transplants with Fusarium wilt may add a new race or strain of the fungus to your field.

Managing these diseases in the field is a different discussion and will be addressed in many articles throughout the year.

 

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