Posts Tagged ‘fusarium wilt’

Watermelon Seedling Diseases in the Greenhouse

Thursday, April 5th, 2012

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Bacterial fruit blotch (BFB)
BFB
of watermelon, which is caused by the bacterium Acidovorax avenae subsp. citrulli, produces large olive green to brown water-soaked lesions on fruit, making them unmarketable. Symptoms of BFB on seedlings are water-soaked areas of the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die.

If the bacterium is present, conditions in greenhouse transplant houses are highly favorable for the development of BFB symptoms and the spread of disease. Good practices for greenhouse transplant production are to disinfect surfaces before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 10,000 seeds. Clean transplant trays (disinfect trays if they will be reused) and new soil must be used. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols, so keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. If BFB is suspected, collect a sample and submit it to your Extension educator, or specialist. Destroy all trays with symptomatic plants. Remove adjoining trays to a separate – isolated – area for observation. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled fungicide and the applications continued until the plants are transplanted to the field.

Olive green water-soaked lesion on watermelon fruit. (Image courtesy David B. Langston, University of Georgia, Bugwood.org)

An inconspicuous lesion of bacterial fruit blotch on a watermelon transplant.

Other Greenhouse Diseases:

Angular leaf spot, which also is a bacterial disease, looks similar to BFB. This “look-alike” disease occurred in Delmarva’s greenhouses several years ago. Symptoms are small dark brown irregular lesions on cotyledons or leaves. Angular leaf spot is favored by cool wet weather. Usually conditions after transplanting to the field do not favor angular leaf spot disease development.

The fungal diseases gummy stem blight, Alternaria leaf blight, anthracnose, and Fusarium wilt can also be introduced into the greenhouse on watermelon seed or through inoculum from a previous crop. Diseases that are transmitted on seed often are randomly located throughout the greenhouse. Initial infections will occur as ‘foci’ or clusters of diseased plants.

Gummy stem blight infected transplants occur as clusters in an area around the initial infected seedling (foci).

Although I have not seen Fusarium wilt infected transplants in local commercial greenhouses, it has occurred in other states. Symptoms are wilted seedlings that may remain green or become chlorotic (yellow). This disease is of special concern because new strains or races can be introduced into an area on seedlings grown from infested seed.

Bottom line: If the seedlings appear diseased, identification of the problem is critical. Do not ship any trays containing plants with disease symptoms.

Fusarium Wilt on Watermelons

Thursday, April 8th, 2010

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu and Emmalea Ernest, Extension Associate – Vegetable Crops; emmalea@udel.edu

Production changes over the past decade have resulted in increasing levels of watermelon Fusarium wilt in Delaware and Maryland. Fusarium wilt is easily recognized by the characteristics of wilting of one vine or the whole plant (Figure 1 A and B) and the red to brown discoloration of the vascular systems (Figure 2).

Figure 1. Single vine runner wilted mid-season (A) and wilted plants at harvest (B).

Figure 2. Vascular discoloration within the cut watermelon stem (Image from A. P. Keinath, Clemson Univ.).

The fungus that causes Fusarium wilt, Fusarium oxysporum f.sp. niveum, was successfully managed for many years through a combination of cultivar resistance in seeded cultivars, rotation, and fumigation. However, the cultivation of seedless cultivars, which have lacked resistance in the past, and the loss of the use of methyl bromide as a fumigant have resulted in an increase in Fusarium wilt. In addition, the pathogen population has shifted to a more virulent type. The good news is that many watermelon breeding programs are working to develop seedless cultivars with resistance to the virulent races present here in Delaware and Maryland. A study conducted at the University of Delaware’s Research and Education Center in 2009 evaluated several seedless cultivars which had been reported to have resistance to race 1. Some cultivars performed well (Table 1) including Abbott & Cobb lines ACR6277TSS, ACX4674T, Seedway’s Sweet Delight and Seminis’ Olympia. We have confirmed that race 2 is present in the field which may have caused the poor results we observed on other lines that also had some resistance. Through a grant from the Delaware Department of Agriculture Specialty Crops Block Grant, we will continue our evaluations in 2010. Stay tuned for an announcement of a field day opportunity to see our results.

Table 1.Watermelon Cultivars Evaluated for Resistance to Fusarium Wilt in 2009

Cultivar Source Wilt Incidence (%)* Yield
7 July 21 July Fruit no. per plot t/ha
Ruby Siegers 36 b** 52 a 3.5 e 3.0 a
Indianna Seedway 68 a 46 a 6.5 de 2.9 a
Melody Seedway 29 bc 27 b 15.0 bcd 8.5 a
Majestic Seminis 13 bc 16 bc 16.8 bc 14.5 a
Sugar Heart Siegers 11 bc 14 bc 16.8 bc 25.0 a
ACX5727 FR Abbott & Cobb 18 bc 13 bc 14.8 bcd 13.3 a
ACR6177TSS FR Abbott & Cobb 5 c 9 bc 20.3 abc 21.7 a
Olympia Seminis 7 c 9 bc 12.5 cde 12.2 a
ACX5117T FR Abbott & Cobb 16 bc 7 c 23.3 ab 31.1 a
Sweet Delight Seedway 7 c 7 c 15.0 bcd 18.7 a
Matrix Seedway 29 bc 5 c 16.5 bc 26.1 a
ACX4674T FR Abbott & Cobb 7 c 5 c 26.8 a 28.9 a
Apollo Seminis 16 bc 5 c 16.8 bc 17.9 a
ACR6277TSS FR Abbott & Cobb 7 c 4 c 22.3 ab 34.0 a
P>F 0.0012 0.0001 0.0012 0.0515

*  Percent of plants that were wilted or dead.

**Mean values in each column and year followed by the same letter are not significantly different at P = 0.05 based on Fisher’s protected least significant different test.

Watermelon Seedling Diseases in the Greenhouse

Thursday, April 3rd, 2008

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

 The fungal diseases gummy stem blight, Alternaria leaf blight, and anthracnose can be introduced into the greenhouse on watermelon seed or through inoculum from a previous crop. To minimize the occurrence of these diseases, the greenhouse should be disinfected before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 1,000 seeds. Use clean transplant trays (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. Keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse.

 As the seedlings develop, inspect them carefully. Infected seedlings will have small brown lesions on the leaves and/or water-soaked lesions on the stem. Diseases that are transmitted on seed often are randomly located throughout the greenhouse. Initial infections will occur as ‘foci’ or clusters of diseased plants. 

gummy stem blight

Gummy stem blight infected transplants occur as clusters in an area around the initial infected seedling (foci).

If the seedlings appear diseased, destroy all trays with symptomatic plants. Remove adjoining trays to a separate area for observation. Monitor these seedlings daily and destroy trays where symptoms develop. Do not ship any trays containing plants with disease symptoms. After symptomatic and adjoining trays are discarded, spray the remaining trays with a labeled fungicide and continue applications until plants are shipped.

 Bacterial fruit blotch (BFB) of watermelon is caused by a bacterium that also is seedborne. Initial symptoms of BFB are water-soaked areas on the lower surface of the cotyledons. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die. Angular leaf spot, which also is a bacterial disease, occurred in Delmarva’s greenhouses several years ago. Symptoms are small, dark brown, irregular lesions on cotyledons or leaves. ALB is favored by cool wet weather. Usually conditions after transplanting to the field do not favor ALB disease development.

bacterial fruit blotch

Small inconspicuous lesion of bacterial fruit blotch

Fusarium wilt also can be seedborne. Although I have not seen Fusarium wilt infected transplants in local commercial greenhouses, it has occurred in other states. Symptoms are wilted seedlings that may remain green or become chlorotic (yellow). No lesions are observed along the stem or petiole but the vascular system is discolored and tan, pink or brown. This disease is of special concern because new strains or races can be introduced into an area on seedlings grown from infested seed.