By Ruth Vaughan
In intensive horticulture soil borne diseases cause yield and quality loss, and if left unchecked can result in complete project failure, as we have seen happen many times.
All soils, unless completely dead, have a large and diverse microbial population, some good and some bad. If things get out of balance the bad guys take control and start damaging your crops.
Pre-emergence damping off
Starting at the beginning when you plant your seeds you can get pre-emergence damping off. This means that the seeds rot in the ground and do not germinate. This condition may be mistaken for bad seed quality. Pre-emergence damping off occurs when conditions are poor: too cold, too hot, too wet, too dry, poor drainage, un-decayed organic matter, high soil salts or compacted soil.
Post-emergence damping off
If the seeds do germinate and grow out of the soil, then suddenly die off, this is called post-emergence damping off. This condition is very often, mistakenly, referred to as dumping off disease. The roots below the soil and the tender stems are attacked by diseases and suddenly ‘fall over’ and die. Stressed plants or weak plants are attacked by the bad guys.
This is caused by all the conditions above and also can be caused by over feeding the plants with nitrogen from too much manure or fertilizer causing very soft leggy growth.
Damping off diseases are caused by a number of funguses, like Pythium, Fusarium, Rhizoctonia and Phytophthora. It is always a good idea before you embark on a costly venture to have a pathology check on your soil, to make sure that these pathogens are not present in the soil, before planting.
Other diseases that are the bain of many a farmer are the vascular wilts. They can come in at any time during the crop growth, when a stress condition takes hold, and enter through the roots. Very often they come in just before harvest, when you have spent a lot of time and money nurturing your crops. Vascular wilts are characterized by plant wilting and discoloration of the vascular system inside the stems. You will see a gradual loss of vigor in the plant, followed by wilting starting at the growing point, yellowing, twig die back leaf, flower, bud and fruit drop, normally ending in sudden death.
Vascular wilts can be of fungal or bacterial origin. Bacterial wilts include Ralstonia, Erwinia, Curtobacterium, Clavibacter, Pseudomonas, Xanthomonas and others. Bacteria breed up in the stem, blocking the vascular tissue and preventing uptake of water and nutrients. They come into and kill the crop very fast. Often if you cut the stem and place it in a clean glass of water you will see a cloudy bacterial stream coming out of the bottom of the stem.They cannot be cured and are catastrophic. The only thing you can do is remove the plants and the soil that they are growing in and burn them.
Fungal bacterial wilts are caused by soil borne fungi and include but are not limited to Verticillium, Fusarium, Rhizoctonia, Pythium etc. If you catch the fungal vascular wilts early – then you may be able to save your crop using a systemic fungicide, although some fungal wilts are incurable (always check the label and make sure the fungicide is PCPB approved for your crop and adhere to the post-harvest interval recommendations.) Generally fungal wilts cause a discoloration inside the vascular bundle and they can all look pretty similar. It is important to send a plant sample for pathology to identify the causal agent. It is important to know what the problem is – both for the possibility of treating your current crop, but also in planning your next crop. Quite often a plant can have a disease complex – which consists of more than one pathogen. These are very hard to treat.
How to prevent soil borne diseases
Crop rotation – this is very important, some diseases affect one plant type and not the other, if you rotate your crop the pathogens do not build up in the soil. Even roses can have a ‘break’ crop of for example Caliente mustard seed is a great short term break crop that will rejuvenate your soil big time. Seeds are readily available in Kenya – search http://shambaza.com/seeds.
Soil Health – make sure that your soil has a good structure and cation balance, plenty of well-rotted compost. Send your soil to the laboratory for a complete soil analysis with lime / gypsum recommendations before planting – this reward you with a good healthy productive crop for many years to come.
Soil Moisture control – very wet soils really, really do cause disease. Some fungal pathogens remain dormant in the soil until the soil becomes too wet when you over irrigate. Wet soil is anaerobic, and many pathogens like anaerobic conditions, so you get more bad guys than good guys building up. Wet soil has no oxygen in it. The plant roots need oxygen to take up water and nutrients. They can’t in wet soil and become weak and prone to disease. Dry soil is just as bad, the plants get stressed and weak and start to wilt, which causes breakages in the roots and stems and entry points for pathogens. “Catch up watering’ aka flooding dry soil is the biggest cause of late onset vascular wilts. Dig done to the roots and really look at the soil moisture to know what’s happening down there. Smell the soil!
Compaction – dig soil pits around your growing area. Compaction causes waterlogging and disease. Compaction breaks roots and causes disease. Compaction limits your root volume causing stressed weak plants that are susceptible to disease. Build raised beds to improve aeration and drainage in the soil.
Nematodes are not a soil borne disease, but the pathogenic nematodes feed on plant roots and weaken the plants as well as creating entry points for soil borne diseases. Very often nematodes are associated with vascular wilts and damping off. It is important to check your soil and treat the nematodes if they are there. The combination of disease spores and bad nematodes in the soil will give you endless problems.
Salinity – high sodium and high EC causes problems with soil and root structure and water infiltration and is a major cause of damping off diseases. A complete soil analysis will let you know what salts are in the soil. Very often the sodium will come from the irrigation water – so it’s a good idea to have that checked too.
High Nitrogen – over application of manure / compost / fertilizer at planting and bed preparation is one of the main causes of damping off disease. Too much of a good thing can be bad. Check the soil and follow the guidelines.
Soil Temperatures – some soil diseases take hold at very high or low temperatures. Over-watering can reduce soil temperatures and mulching can improve them. A happy diverse and active soil can ‘warm’ itself up.
Beneficial micro-organisms – having a lot more of the good guys than bad guys in the soil can make a huge difference to disease pressure. The beneficials work in many varied ways, they feed on the baddies, they out compete them, they strangle them, they recycle your plant nutrients and make them more available – creating a stronger healthier plant. Beneficial microbes are easily available in Kenya – money spent on these at planting, will save you chemical bills and pesticide residue problems later on.
If you have a problem with persistent plant diseases or want to get your project off to a good sustainable disease free start please feel free to contact our agronomists on firstname.lastname@example.org and see what we can do to help you!