Blight means anything which obviously damages (the crop), by rain, hail, locust, blast of wind, or fire.  

الْجَوَائِحُ كُلُّ ظَاهِرٍ مُفْسِدٍ مِنْ مَطَرٍ أَوْ بَرْدٍ أَوْ جَرَادٍ أَوْ رِيحٍ أَوْ حَرِيقٍ  

Blight is not effective when less than one-third of goods are damaged. Yayha said: That has been the established practice of Muslims.  

أَنَّهُ قَالَ لاَ جَائِحَةَ فِيمَا أُصِيبَ دُونَ ثُلُثِ رَأْسِ الْمَالِ قَالَ يَحْيَى وَذَلِكَ فِي سُنَّةِ الْمُسْلِمِينَ