6

Up to this date we have had a healthy crop of about 500 flowering pumpkin plants, however, over the last 3 days we have lost maybe 20 of them. They will simply wilt over 24hrs and then die:

enter image description here

Closer inspection shows that the base of the stem is brown/damaged and in many cases hollow - this particular plant actually appeared healthy (no wilting yet), but its stem broke very easily:

enter image description here

Opening up the root and stem we see no pests, larvae, etc. but similar presentation over and over:

enter image description here

Personally, I suspected it may have been wind damage from a recent storm, but I would expect to see more obvious damage in other areas.

Cutworms are present in the soil, but we have only ever seen them kill much smaller plants (which we now protect):

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There are these guys hanging about also, but not many of them:

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Finally, internet searches lead us to fungal crown rot, but that seems odd in that the field in question has been used for cattle grazing for years before this happened. We thought we'd ask here so as not to rule out anything else, and if it is crown rot or otherwise:

  • What if anything, can we do to fix this now, this year?

Organic solutions are preferred.

  • Are the roots affected at all or is it just the crown? Are there any patches of mycellium visible near the stems in the soil? – Graham Chiu Jan 12 '18 at 18:42
  • And where did your source your seed from? And was it certified free of fusarium? – Graham Chiu Jan 12 '18 at 18:48
  • @GrahamChiu, Thanks for your question. We think it's just the crown, the photo of the root above is pretty typical. My partner says she didn't spot any white residue on the soil or stem (assuming that is an indicator of mycellium/fusarium?). The seeds we purchased from an organic supplier with no indication that they were fusarium free. – Lamar Latrell Jan 12 '18 at 19:51
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    I'd ring the supplier to check about others with this problem. Since your soil wasn't growing curbits within the last 2-3 years the infection must have come with the seeds. – Graham Chiu Jan 12 '18 at 19:53
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    The two different larvae you're showing are leatherjacket (the brownish grey one) and a chafer grub... they'd have eaten the roots if they were the problem. The insect in the final picture is a 5th instar of a southern green stink bug (Nezara viridula) - they do damage crops, but they usually choose softer parts like new stems and leaves, not the tough mainstem. I agree its probably fusarium... – Bamboo Jan 13 '18 at 0:05
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From the images and description, this most likely represents an infection by Fusarium solani f. sp. cucurbitae.

enter image description here

The first symptom usually noticed in the field is wilting of the leaves. Within several days, the entire plant may wilt and die. If the soil is removed from around the base of the plant, a very distinct necrotic rot of the crown and upper portion of the taproot is evident (fig. 5). The rot develops first as a light-colored, water-soaked area which becomes progressively darker. It begins in the cortex of the root, causes cortex tissue to slough off, and eventually destroys all of the tissue except the fibrous vascular strands. Infected plants break off easily about 2-4 cm below the soil line. The fungus generally is limited to the crown area of the plant. The main and lower portions of the taproot are not affected, except under extremely wet conditions. Likewise, the stem is not affected, except for the lower 2-4 cm immediately above the soil line, as seen in experimentally inoculated Howden pumpkin seedlings (fig. 6). Plants showing symptoms develop numerous sporodochia and macroconidia (spores) giving the mycelia a white to pink color on the stem near the ground surface (fig. 5). Fruits are attacked at the fruit-soil interface; the severity of the fruit rot depends on soil moisture and the stage of rind maturity at the time of infection (fig. 7). source

Since the spores of this fungus can only persist for 2-3 years, and your soil has not grown curcubits in that time period, then the most likely explanation is that you have purchased contaminated seeds since Fusarium free seeds are not generally available. Infection is less likely if you use fungicide treated seeds, and practice a 4 year rotation.

You should pull the affected plants and destroy them, and perhaps even remove the soil around the plants though that may not be practical.

As to why a seed borne disease affects an adult plant I'd speculate that because it does not affect the fibrous vascular strands it still allows the plant to grow to reach a certain point until it can no longer sustain the plant.

Fusarium Crown and Foot Rot of Squash and Pumpkin Fusarium solani f. sp. cucurbitae Race 1

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Squash Vine Borer

This is similar to what happens with cabbage, broccoli, cauliflower...they have their own 'borer'...a beetle. Others like this are flies, moths. They lay their eggs at the base, when the eggs hatch, the larvae eat the roots and crown of the plant. One day it just falls over or wilts unexpectedly.

There is nothing you can do now. I would check all my plants. You'll be able to see if the plant has been compromised by this borer. If you find healthy plants, leave them alone. They are not in trouble.

There is a particular window of time where these insects are laying their eggs at the bottom of susceptible plants. I use row cover. Light, transparent cloth that if you close all the edges with soil, will prevent this happening again. This article should tell you when to expect that window. The parents are gone. When the larvae mature...they fly away. There is nothing you can do now.

Harvest your squash now, hopefully you'll have a few viable plants and really that is all you need for a great squash harvest. Next year make sure you have row cloth. I would cover my squash, and brassicas with row cloth as soon as they are planted as starts or allowed to get 4" high from seed. Poof up the cloth so there is plenty of air circulation. Water well before installing the cloth...soak your soil. Use a few sticks to help poof up the cloth then tuck every edge into the soil so no insect is able to fly beneath and lay their eggs. Water will penetrate the row cloth but you still need to check that moisture is getting down at least 2". Remove the cloth when the window of time is up...I think this article will tell you that. Depends on your local, zone, temperatures...

  • the images show cortex necrosis leaving the exterior of the stem intact so this isn't external insect damage. – Graham Chiu Jan 13 '18 at 18:47
  • Hey, this was a great question and answer, Graham. Is there anything he can do other than fungicide? – stormy Jan 13 '18 at 21:45
  • From what I read, even fungicide is now too late since the fungus is inside the plants having come from inside the seeds. He needs to pull the plants, and remove the affected soil. – Graham Chiu Jan 13 '18 at 22:28
  • Graham, that is also what I know. Fungicide is to be used as a preventive not at all a remedy...except for powdery mildew. Removing the soil I don't think is even rational. Just ONE spore is all it takes to do this all over again. What do you know about solar cooking of the soil? Is it hot enough? This is with clear thick plastic, not black. They say 4 years? I don't think I would ever want to use that soil again. Are there soil tests for Fusarium, blights, bacterial infections? Graham...five hundred plants? This is indeed a catastrophe. – stormy Jan 14 '18 at 5:30
  • The spores can only persist for 2-3 years. So, that's why they suggest a 4 year crop rotation if you have infected plants. – Graham Chiu Jan 14 '18 at 6:23

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